First seen on the screen in the Star Trek (original series) episode Balance of Terror, the Romulans are notoriously devious and tricky.
The Romulan species is actually a splinter group of Vulcans who refused to accept Surak's teachings of completely suppressing ones emotions. Around 400ce they split off from Vulcan society and established colonies in the Romulus system with home planets Romulus and Remus.
In the mid 1900s the Romulans and Vulcans entered into war, lasting until 2044. Later in 2150 the humans make first contact with the Romulans and four years later the Earth-Romulan war began.
In 2160 the Romulans and the humans sign a treaty establishing a neutral zone one light year-wide between their territories and in 2161 the humans formed the United Federation of Planets.
About six years later the Romulans invented a new piece of technology called a cloaking device which made their ships invisible to sensors. They then preceded to break the treaty by attacking several Federation outposts. In reply to these attacks, the USS Enterprise tracked down the ship carrying out the attacks and destroyed it.
Roughly five years later, in 2267, the Romulans entered a treaty with the Klingon Empire, and received several D7 battlecruisers (Birds of Prey) in exchange for cloaking technology.
In 2293 the Klingon moon Praxis exploded and the Klingons entered into a treaty with the Federation. This ticked off the Romulans who, in 2311, attacked the Federation. Following the attacks the Romulans and the Federation created a new treaty, the Treaty of Algeron, which reestablished the neutral zone and prohibited the Federation from developing cloaking technology and there were no further incidents for fifty years.
In 2344 the Romulans attacked the Klingon outpost Narendra III. The USS Enterprise-C was destroyed while defending the outpost, thus strengthening relations between the Klingon Empire and the Federation and initiating a war between the Romulans and the Klingons, which lasted several years.
In 2366 the Romulans attempted to trick the USS Enterprise-D into crossing the neutral zone where they have several Romulan Warbirds waiting to destroy the Enterprise. Captain Picard, however, brought along four Klingon Bird's of Prey for backup and the Romulans retreated. (The Defector)
The Romulans then, in 2367, attempted to brainwash Federation lieutenant commander Geordi LaForge but the plan was exposed and failed (The Mind's Eye).
Later in 2367 the Klingon Civil War began and the Romulans secretly backed the house of Duras, but Captain Picard of the Federation discovered this and convinced Federation-friendly Gowron to attack the Duras forces hard in hopes that they would request the Romulans bring in reinforcements, which would be detected by a Federation blockade, thus revealing to everyone that Duras was backed by the Romulans. The plan worked and Duras quickly lost all support for the Chancellor's seat. (Redemption, Part I)
Sometime during or before 2368, Ambassador Spock, a Vulcan, was on Romulus, working with an underground movement for the reunification of the Romulans and Vulcans. The Romulans, however, saw this as a chance to conquer the Vulcans and sent a fleet of ships to Vulcan, but their intent was discovered and the fleet destroyed (Unification, Part I, Unification, Part II).
In 2371 the Romulans allowed the Federation to borrow a cloaking device under the condition that they only used it in the Gamma Quadrant, where a new threat is emerging with Jem'Hadar and the Dominion. (The Search, Part 1, The Search, Part 2)
The Romulan intelligence agency, the Tal Shiar allied with the Cardassian equivalent, the Obsidian Order, in late 2371 and launched a preemptive strike on the Dominion, but the Dominion destroyed them both and it was revealed that General Lovok, the Tal Shiar officer leading the attack was in fact a Dominion agent (The Die Is Cast).
The Klingons went a bit kill-everything-in-sight crazy then and attacked the Cardassians and several Romulan outposts. The Romulans sent a warbird to join the Klingon and Federation front guarding the Bajoran wormhole but when a Founder disguised as Doctor Bashir made a failed attempt to blow up Deep Space Nine, the Romulans signed a nonaggression treaty with the Dominion and went home.
In 2374 Captain Sisko and Garak showed Romulan Senator Vreenak fake recordings of a Dominion strategy meeting where it's revealed that the Dominion intends to attack the Romulans after taking the rest of the Alpha Quadrant. Vreenak examines the recordings and discovers they're fake and leaves the station to inform the Romulan government. (Bad doo-doo for Federation-Romulan relations.) Garak, however, rigs Vreenak's ship with a bomb and it explodes, killing Vreenak and when the Romulans recover the recording from the wreckage the defects that allowed Vreenak to discover their fraudulence are assumed to have been caused by the explosion, which is blamed on the Dominion. This, of course, is all top secret and only two-three people are aware that the tapes aren't real. (In the Pale Moonlight, one of the best episodes ever).
Now that the Romulans have joined the Klingon-Federation alliance, they establish a presence on Deep Space Nine in 2375 and attempt to stockpile weapons on the Bajoran moon. However, Colonel Kira stops them with a Bajoran fleet, with the help of Federation Admiral Ross's statement that the Federation will back the Bajorans in this matter (Shadows and Symbols).
In 2375 Section 31 attempts to assassinate the Romulan Koval, the new head of the Tal Shiar. The plan is stopped when Bashir, who is being used by Section 31 as a spy, asks Senator Cretak for assistance in stopping the plot (Inter Arma Enim Silent Leges).
In 2375 the Federation-Romulan-Klingon fleet heads to Cardassia Prime to finish off the Dominion's presence in the Alpha Quadrant. The fight isn't going well until the Cardassian fleet turns and joins the fight against the Dominion. End of War (What You Leave Behind).
Several other websites for very minor details.
My writeups for Martok, Worf, Cardassians, Geordi LaForge, Klingon and What You Leave Behind
This writeup is largly from memory. I am a Trekkie.
This writeup is licensed under the GNU Free Documentation License.