The short version is that Ukraine has long been split between folks who look West toward Europe and those who tend to lean East toward Russia. Last year, now-ousted President Viktor Yanukovych decided to scrap a European trade deal that promised long-term rewards in favor of a pile of quick cash offered up by Russia. The pivot toward Russia set off protests by the pro-European crowd that climaxed in deadly clashes with security forces last week. Dozens died before a European-brokered peace deal ended the fighting. But Yanukovych fled before the deal could all be implemented, first to pro-Russian strongholds in Ukraine, then finally to Russia.
Ukraine's Parliament says it's running things. Lawmakers have named an interim president, started to set up a new government and called elections for May. They've also been talking tough to Russia about keeping its military on its leash and avoiding anything that seems provocative.
Why does Russia care? Well...Why does the United States care what's happening in, say, Mexico? Pretty much the same reasons apply -- the two countries have traded territories back and forth, they have a shared border and there are lots of business and cultural associations. Ukraine is literally on Russia's doorstep. It's home to the country's Black Sea naval fleet. Ukraine used to be one of the republics in the Soviet Union, and Crimea used to belong to Russia. The countries are big trading partners, and Ukraine hosts part of Russian state-owned natural gas provider Gazprom's network of pipelines that serve European markets.
Keep an eye on Crimea. Pro-Russian forces still hold several government buildings, and lawmakers there have voted out a government they saw as too pro-Kiev. With so many ethnic Russians and a big Russian military presence there, it's a potential flashpoint.