Nicholas I was the third son of Emperor
Paul of
Russia and was born in
1796. He was not considered likely to take the
throne since he had two older brothers, and was brought up to be a
soldier. Unlike his older brothers, he was not tutored in
Enlightenment philosophies by their grandmother
Catherine the Great, as she died not long after Nicholas was born, so he was generally more
conservative than
Alexander I, who became Emperor when Paul was assassinated in
1801. Nicholas' mother tried to wean him away from the military subjects he enjoyed so much but never succeeded.
Neither Alexander or second-oldest Constantine had sons, so Nicholas was named the heir to the throne in 1819, but when Alexander died in November 1825, this had not been made public -- in fact, Nicholas did not know it had been done until after Alexander's death. So the country was prepared to swear allegiance to Constantine, including Nicholas, while Constantine was writing letters from Poland renouncing his right to the throne. Nicholas was not popular among some Army Guard members, and a secret society, later to be called the Decembrists, came out in support of the more easy-going Constantine, who they thought might be forced to proclaim a constitution, and persuaded many soldiers that Constantine was the legitimate Emperor. Battles between loyal and rebel soldiers took place in Moscow, and Nicholas led a battalion of his supporters. The Decembrist uprising made Nicholas suspicious of people throughout his reign, and he took steps to squash all opposition.
Nicholas did, however, try to improve the situation of serfs in Russia; he did not approve of serfdom, but like the last few rulers, felt that the nobles would not let him free the serfs. He also built railroads, tried to increase technology and industry in Russia, and published a collection of Russian laws, which had not been available all in one place since 1649. Nicholas was in favor of the Orthodox Church and religious education instead of secular schools. He was also a patron of the theater, and loaned the poet Aleksandr Pushkin money to publish a historical poem.
The big political questions during his reign were the status of Poland, a "kingdom" under the dominion of Russia, and "the Eastern Question," the desire of getting access to the Mediterranean Sea (through the Black Sea, whose Mediterranean outlet was controlled by the Ottoman Empire). After a Polish rebellion in 1830, Nicholas revoked the constitution of Poland and tried to integrate it into Russia.
The Eastern Question was more difficult, as the Ottoman Empire included a lot of Orthodox subjects who wanted Russia to free them from Muslim domination. This complicated Russia's more simple desire to be able to send ships into the Mediterranean through the Black Sea. In 1829, an English-French-Russian alliance defeated a Turkish-Egyptian fleet and forced the Ottomans to recognize Greek independence and give some Black Sea coast to Russia, but increasing Russian power led several European powers to oppose Russian forces in the Crimean War starting in 1853. Under a lot of stress, Nicholas fell ill with pneumonia in 1855 and died on February 17, with his oldest son Alexander II succeeding him. (Some rumors say Nicholas committed suicide by poisoning himself, but this seems out of character.)
Sources: Donald Raleigh and A.A. Iskenderov's The Emperors and Empresses of Russia: Rediscovering the Romanovs and those listed in Monarchs of Russia.