Alright this is going to be a bit lengthy lol but here goes:
Ukraine used to be a part of Russia as the USSR up until the late 80's-early 90's. They speak the same language, their cultures are basically identical, etc. But Ukraine has always felt that they should be their own country and thus got their independence when the USSR broke up.
To the south and southeast of Ukraine are several small semi-autonomous regions called Crimea, Dagestan, and Chechnya. Chechnya and Dagestan are part of the Russian Federation. These areas are part of Russia but have their own independent laws and governors like the states in America. They don't like Russia and want to be fully independent as well; Chechnya commits acts of terror in Russia and has rebels fighting the Russian Military constantly. They were also responsible for the bombing at the Boston marathon last year.
Crimea is a part of Ukraine but is again semi-autonomous. Its population is divided between Russian Nationalists and Pro-West Crimeans. The majority is Pro-Russia and thus they have been clamoring for Russian intervention for years and to be nationalized under Russia once more. There has been a lot of tension and violence in the region, with the Nationalists being attacked for years.
Ukraine recently had a very corrupt and useless prime minister who was run out of the country several months ago, destabilizing the region. Russia saw this as an opportunity to 1. Take Crimea back. 2. Perhaps try to reunite the USSR? (the jury is still out on this one) So Russia's military rolled in to Crimea and held a vote (with probable military intimidation) that found the majority wanting to become a part of Russia again. The Crimean populace seems happy overall with the decision. Now Russia is on the border of Ukraine threatening to invade though its all posturing at this point and I doubt they will actually go through with it.
So our final point is the dilemma: Ukraine is Pro-West/Europe which really hasn't gotten them much. The West considers them very low priority though Ukraine does benefit from the big business of Western capitalism. However, Russia dominates Asian, Middle Eastern, and European supply of oil and natural gas. They have huge pipelines that go everywhere. So Europe isn't coming to Ukraine's aid and for once the US isn't either.
Russia is stupid. Their country is in complete shambles and has been for years. Their people are starving and they spend a fortune on hosting the winter olympics. Its idiotic.