The woman who would later be Catherine the Great was born Sophia Augusta Frederika of Anhalt-Zerbst, a minor principality in Germany
(now part of Poland
) in 1729
to a father, Christian August, who was happy to serve Frederick II of Prussia
and a mother, Johanna Elizabeth, ambitious to be more influential than her husband's position would let her. Growing up, young Sophia was largely ignored by her mother in favor of her younger brother. This changed in her mid-teens.
Empress Elizabeth of Russia in 1741 had seized the throne from her cousin Anna and the baby Tsar Ivan VI, who were descended from Peter the Great's brother. Elizabeth had once been engaged to Johanna's brother, but he died before they could marry. She never married, but adopted her nephew Peter as her heir (much to his dismay, as he preferred Prussia to Russia). When Peter was in his late teens, Elizabeth decided it was time for him to marry and produce an heir; she thought of Sophia, the niece of her late fiance.
Johanna and Sophia were summoned to Russia in 1743. They got along with Elizabeth quite well, and Sophia and Peter became friends because Sophia was willing to humor this neurotic teen and play toy soldiers with him. Sophia converted from Lutheranism to the Russian Orthodox Church and was baptized Catherine Alexeyevna. Peter, however, came down with smallpox and afterwards, his face was disfigured with scars; Catherine is supposed to have flinched from him the first time they met after his recovery, nipping the friendship of the engaged couple in the bud.
But Elizabeth left them no choice; the two married in 1745. It is supposed to have been several years before the marriage was consummated, and Elizabeth was wild to find some way to get an heir to the throne from these two (she didn't really consider Peter ruling material anymore). When Catherine became pregnant in 1754, the story went around at court that Peter had phimosis, a medical problem where the foreskin won't retract, and that it was finally corrected by circumcision, but this may have been an attempt to cover up the fact that at Elizbeth's desperate encouragement, Catherine had taken a lover, Serge Saltykov.
Paul was born (and who his father was is still in doubt) and was taken away from Catherine by Elizabeth, who wanted to raise him herself. Catherine, largely abandoned by husband and empress now that she'd done her duty of producing a successor, filled the next few years largely with reading, and absorbed all the Enlightenment ideas she was later to use as a ruler.
Peter was getting steadily more unpopular with both aristocracy and others in Russia; he made no secret of his hero worship of Frederick of Prussia. He also insulted the church and became known for drunken misbehavior, flaunting his mistresses. Catherine, on the other hand, had adopted all things Russian to her heart, and spent as little time with Peter as possible, disassociating herself from him in the public eye.
Elizabeth died in 1761 and Peter was crowned Emperor Peter III. He immediately stopped the ongoing war with Prussia that was draining the treasury, which would have been a popular move except that he drew up a treaty in which Russia was treated as the submissive loser of a war it had been winning. Even Frederick of Prussia, who wanted this kind of friend to stay in power, couldn't persuade Peter to do things that were going to get him support in Russia.
Catherine endured Peter's mistreatment (calling her a fool at the dinner table at a banquet, etc.) patiently and won great public respect for it. She really had to wait, though, until her third child was born (secretly -- this one was certainly the child of her current lover Gregory Orlov) and given away before she could act, and even after that she continued to bide her time until Peter did something that forced her hand.
In addition to being Emperor of Russia, Peter was also ruler of Holstein, now a part of Germany. As soon as the Russian troops were free from the Prussian conflict, he proposed to march them against Denmark to win some land he said was really part of Holstein. No one else in Russia cared about Holstein, and the country couldn't really afford another war. Peter became more unpopular than ever and conspiracies popped up everywhere -- some wanting to rescue imprisoned Ivan, a few to make Catherine regent until Paul reached adulthood. Peter was suspicious, and when he arrested one of Catherine's friends with whom she had been making plans to overthrow Peter, she had to act to save herself from being arrested.
Catherine went with her lover Orlov (who was a soldier and had been working among Russian soliders in support of Catherine) and asked for "protection" from Peter. They burst out in cheers and their chaplain came out and proclaimed her "sole ruler of Russia." The group, swelled by every regiment of soldiers they encountered, made their way into St. Petersburg and were cheered by the populace. News eventually reached Peter, who was on his way to a country retreat to celebrate St. Peter's Day, but he was torn by indecision and did not take the opportunity to rally the soldiers Catherine had not yet reached (though he could have if he had made a decision to do so quickly). Without bloodshed, Catherine was able to arrest Peter and make him copy out an abdication she had worded. He was imprisoned (at first in the same city where Ivan still languished in prison).
Catherine probably did not have Peter murdered, but he died at the hands of his guards not too long after being arrested. The letter Catherine received with the news of Peter's death from Alexis Orlov (brother of her lover Gregory) read in part: "He quarrelled with Prince Feodor at table; we could not separate them, and already he was no more! We ourselves cannot remember what we did, but we are all guilty to the last man and deserve death. Have mercy upon us, if only for my brother's sake!" (Somehow this drunken fight seems like an appropriate way for Peter to die.) This letter was enough to convince Paul after his mother's death the she hadn't conspired for Peter's murder. However, she did not do anything to punish his supposed guards.
Ruling was not easy. Catherine tried to free the serfs who worked in factories and mines, but the riots the freed serfs caused forced her to send in troops to subdue them. She had to deal with a southern peasant claiming to be Peter, whose rebels controlled a third of Russia for a while. There were also more conspiracies to put Ivan on the throne -- neither Elizabeth nor Catherine had been willing to just execute him without provocation, so he was still in prison. (He finally died when a soldier tried to break him out of prison; Ivan's guards had been given instructions to kill him rather than let him go free.) And she had to deal with Orlov, who wanted to marry her, which was something the nobles of Russia would never accept. Gradually Orlov fell from her favor (though she said it was because he became tired of her) and in 1774 another soldier, Gregory Potemkin, became her lover.
Potemkin was probably more truly in love with Catherine than any of the other men she was involved with. Even though their romantic relationship only lasted two years before they separated and Potemkin went off to fight for Russia in its southern regions and to try and conquer them a path to the Black Sea. Most of Catherine's lovers after this were hand-picked by Potemkin, and dumped when he told her to leave them. (She is "notorious for her illicit affairs" only because she was a woman -- male monarchs of Europe generally had as many or more lovers in a similar time period.)
In 1783 Potemkin was able to conquer the Crimea for Russia, and he and Catherine planned to conquer Turkey from the Ottoman Empire next (with help from Austria, which would receive much of the European territory then occupied by Turkey.) Potemkin sent glowing reports of how the new provinces were prosperous. But when Catherine came to see for herself, bringing representatives from other countries, Potemkin took the travelers' carriages through villages that were just facades with peasants shipped in to look content. Potemkin never was able to conquer Turkey, even though he tried in every way to win Catherine's favor. He is supposed to have died from eating a meal of food his doctors had forbidden him to have, after a fight with Catherine who refused to give up Zubov, her last favorite, on his command.
Catherine was also distracted by the French Revolution, which despite her Enlightenment ideas she disapproved of after her own experience with trying to free the serfs, and other European politics. Russia, Austria and Prussia pretty much carved Poland into pieces in the 1790s.
When she died (of a stroke in her bedroom or bathroom, no horse involved), her son Paul (Pavel) became Tsar. He hadn't gotten along with Catherine well, and was eerily like Peter for someone who might not have been Peter's son. He reigned for five years, until 1801, and was succeeded by his son Alexander I, who Catherine had partially raised and taught her ideas on ruling, in hopes that over time things could be better for the people of Russia.
Sources: Gina Kaus' Catherine the Great: Portrait of an Empress, The Big See for the phimosis information, and various Web sites.