Short answers, pulled from the story.
The Decembrist revolt was a failed coup attempt by liberal military officers and political dissidents against the Russian Empire, carried out in Senate Square in Saint Petersburg in December 1825. Around 3,000 soldiers refused to swear allegiance to the incoming Emperor Nicholas I, demanding instead that Konstantin take the throne. Government forces crushed the uprising the same day using artillery fire.
The Northern Society, led by officers including Nikita Muraviev, Prince S. P. Trubetskoy, and Prince Eugene Obolensky, organized the Senate Square uprising. Prince Trubetskoy was elected interim ruler but abandoned the square on the day of the revolt; Prince Obolensky replaced him in the field. In Ukraine, Sergey Muravyov-Apostol led the Chernigov Regiment in a separate uprising that began on the 10th of January 1826.
Five principal leaders were hanged. The court also sentenced 31 men to beheading (commuted), and handed down punishments ranging from cashiering and lifelong exile with hard labor to shorter exile terms and demotion to the ranks. About 120 more received extrajudicial punishment, and around four thousand enlisted soldiers were sent to the Caucasian front. Many lower-ranking participants received thousands of lashes before being marched to Siberia on foot.
Most Decembrists were sent to Siberia, with the largest group initially held at Chita in Zabaykalsky Krai before being transferred to Petrovsky Zavod near Nerchinsk. Fifteen were dispersed to remote settlements including Berezov, Narym, Yakutsk, and Vilyuysk. Conditions involved strict police surveillance, censored mail, and meager land allotments of roughly 16 hectares. Commandant Stanislav Leparsky at Petrovsky Zavod largely failed to enforce hard labor requirements, and most Decembrists left the facility between 1835 and 1837.
Many wives of Decembrists gave up their social rank and comfort to follow their husbands to Siberia. Under the leadership of Princess Mariia Volkonskaia, they waged a sustained petition campaign and by 1832 had secured the formal cancellation of labor requirements and the right for husbands to live privately with their wives. Maria Volkonskaya followed Sergei Volkonsky to Irkutsk, where she founded schools, a foundling hospital, and a theater. The Russian word Dekabristka, meaning Decembrist wife, became a symbol of spousal devotion.
Russia's autocracy continued for nearly a century after the revolt, but serfdom was officially abolished in 1861 and parliamentary institutions arrived in Russia and Finland in 1905. Mikhail Speransky, a reform advocate who had been forced into exile under Alexander I, was appointed by Nicholas I in 1826 to head a committee that codified Russian law and produced a complete collection of 35,993 enactments. The Decembrist uprising is widely regarded as the first open breach between the tsarist government and reformist elements of the Russian nobility.