I wrote this on Ukraine Issue:
Regionalism in Ukraine, and the role of Russia in that regionalism, has largely led to the current crisis in Ukraine. Southern and eastern Ukraine, where Russian tends to be the primary language (even though the majority are ethnic Ukrainians), the people favor warm relations with Russia.
In central and western Ukraine, where Ukrainian is the primary language and people harbor more Ukrainian nationalistic views, there is a strong desire to join the European Union.
Yanukovych won the 2010 elections, and though he courted integration with the EU, in the end he and his party balked (some believe due to Russian pressure), which touched off the protests, the riots, and eventually (especially after the brutality of the Berkut and the abuse of demonstrators which led to large numbers of people killed or wounded) the rebellion.
The rebellion led to in essence a coup that saw Viktor Yanukovych and his party effectively bounced from power. This no doubt has led to hard feelings from southern and eastern Ukraine...but in Crimea, the only area of Ukraine where ethnic Russians constitute a majority, and where the Russian military has a sizable presence, it seems the view is that the region could be taken away from Ukraine with little risk of complications.
As for whether Ukraine will go to war with Russia...I think Crimea is the special exception. It is an autonomous region of Ukraine, the only one where ethnic Ukrainians (at around 24% of the population) represent a minority, and where the presence of Russian troops would've made it difficult to defend if/when Russia wanted to seize it anyway.
If Russia moves on any of the oblasts, even the ones that tend to be pro-Russian like Luhansk and Donetsk, I don't think Ukraine will have a choice: it will have to go to war, or it will lose either its nominal independence or its vary existence.
"Why there president was ousted?"
He's blamed for the deaths and injuries of demonstrators and the abuses of the Berkut.
"Why Russia is taking over Cermia?"
The primary reason is a warning to Ukraine: don't step out of line, or we'll step on you. If Ukrainian nationalists don't back down, my suspicion is that Crimea will never be returned to nominal Ukrainian sovereignty.
Another key reason is because that's where Russia's Black Sea naval fleet is located.
The third reason, which is trumpeted as the first but undoubtedly not...is the ethnic Russian majority in Crimea. This is trumpeted as the primary reason, but it's a wholly invented reason because no one really believes that ethnic Russians are at risk of being victims of violence at the hands of ethnic Ukrainians!