Population of Ukraine is split into two parts. Roughly by the Dnieper river.
These parts have very different history. They speak different languages (most of them).
Shortly the story goes like this.
Western Ukraine has its origin in Kievan Rus. Soon after Mongol invasion, part of this territory joined the Kingdom of Poland, another part the Great Duchy of Lithuania.
Later Poland and Lithuania united in one state called Polish-Lithuanian Commonwelth.
However the population of the present Western Ukraine never mingled with the rest of
the population of the Commonwealth because of the religious difference.
(Ukrainians were mostly Orthodox, Poles and most "Lithuanians" Catholic).
Modern Belorussia was also a part of the Commonwealth.
The territory of Eastern Ukraine (Wild steppe) was settled much later,
by settlers from Russia and from the Commonwealth (Cossacs). Until 18th century
this was a nomad territory, controlled by various Tatar descendents of
the Mongol state.
As a result of 17th century wars it went to Russia, and the territory of
the modern Ukraine was split along Dnieper between Russia and the Commonwealth.
Polish-Lithuanian Commonwealth was destroyed in the end of 18th century, jointly
by Russia, Prussia and Austria. As a result, most of the Ukraine "united"
within Russian empire, except a small part which remained in Austria.
After WWI, this part went to Poland and Roumania, and in 1939-40 Soviet Union invaded Poland and Roumania,
and joined this remaining part to Soviet Ukraine.
So there was always a tension, within Ukraine: one part of the population feels
"European" and another feels "Russian". One historian noticed that the dividing line
almost exactly coincides with the dividing line between steppe and forest geographical zones:-)
The most radical pro-European part is exactly that one which was annexed by Soviet Union
in 1939-40. (Lviv, Ternopol regions). The most pro-Russian are eastern regions
of Kharkiv, Donetsk, etc.
A separate part is Crimea peninsula.
Its population was Tatar. It was invaded and annexed by Russia in 18th century.
In 1940-s the Soviets expelled all Tatar population from Crimea.
Only after collapse of Soviet union they were permitted to return.
Crimea was administratively joined with Ukraine in the second half of 20th century, though historically and geographically it was always closer to Ukraine then to Moscowia.
Most of the non-Tatar population nowadays speaks Russian.
It is very questionable which part Crimea will take in the current conflict.
(Added in 2020: the doubts about this completely disappeared in 2014: majority of Tatar population is on Ukraine's side).
This, to my understanding explains the differences in Ukraine.
I don't give any references, all this information can be easily checked with Wikipedia.
(Or you may consider this first-hand account as I lived most of my life in Ukraine,
in both parts of it:-)
EDIT. This was written more than a year ago, and dramatic changes happened in this period, so I will update. One can shortly say that during the last year Ukrainians finally consolidated in a modern nation. In the face of the Russian aggression, the differences between the East and West, and also between ethnic Ukrainians, Tatars and Jews, as well as language and religious differences became secondary: an overwhelming majority of citizens of Ukraine feel themselves as one nation now. Unfortunately, one has to thank Russian invasion for this unexpected development.