It all began as an internal Ukrainian crisis in November 2013, when President Viktor Yanukovych rejected a deal for the chance to get closer with the European Union sparking mass protests, which Yanukovych attempted to put down violently. Russia backed Yanukovych in the crisis, while the US and Europe supported the protesters.
Since then, several big things have happened. In February, anti-government protests toppled the government and ran Yanukovych out of the country . Russia, trying to salvage its lost control in Ukraine, invaded and area known as Crimea the next month. In April, pro-Russia separatist rebels began taking territory in eastern Ukraine. The rebels shot down Malaysian airlines flight 17 on July 17th, killing 298 people, probably accidentally but not for certain. Fighting between the rebels and the Ukrainian military intensified, the rebels started losing, and, in August, the Russian army invaded eastern Ukraine to support the rebels. This has all brought the relationship between Russia and the West to its lowest point since the Cold War. Sanctions are pushing the Russian economy to the brink of recession, and more than 2,500 Ukrainians have been killed.
A lot of this comes down to Ukraine's centuries-long history of Russian domination. The country has been divided more or less evenly between Ukrainians who see Ukraine as part of Europe and those who see it as intrinsically linked to Russia.
Meanwhile, in Russia, Putin is pushing a revival, spreading a nationalist worldview that sees Ukraine as part of greater Russia.