The Crimea was and has been an autonomous region of the Ukraine in the northern Black Sea. Most people living in Crimea are ethnically Russian, however the Crimea itself is not part of Russia, many living within the region wish to break from the Ukraine and officially join Russia, although it is unclear how truly strong this sentiment is as the United States is claiming that people in the Crimea are being intimidated by Russia's leader Vladimir Putin. Here is a little run down:
In November of last year Ukrainian President Viktor Yanukovych broke off discussions with the European Union in order to gain favor with Russia. This decision led to protests by pro-European groups. Over the next few months protests got more and more violent until finally in February protesters demand a return to the 2004 constitution which limited the powers of the Ukrainian President.
On February 22 Yanukovych is impeached and he flees the country. Following this the leader of the opposition movement Yulia Tymoshenko is released from prison and Parliament elects Tymoshenko's friend Oleksandr Turchynov as interim president. Turchynov is reject by Russia and Pro-Russian forces begin to demonstrate in the Crimean region until on February 27 armed gunmen seize the government buildings in Crimea, raising a Russian flag over the region.
Since then, on March 16 there was a referendum which resulted in 97% of people in the Crimea voting to leave the Ukraine in favor of joining Russia.
This is seen as a power play by Vladimir Putin. The West claims that Putin has staged the event in order to gain territory and influence. President Obama and Secretary of State John Kerry have made statements declaring that the world will not standby while Putin annexes lands and territories....
...and so here we are in a Cold War like standoff with our old friendly rival.. Russia.