The Eastern and Southern Ukraine are mostly Russian-speaking. Although Ukrainian is official throughout the country, and many street signs are in Ukrainian, the people use Russian there in their daily conversations. I visited Donetsk region, Kharkiv, Crimea, Odessa and Kiev. All those places are mainly Russian-speaking. Even in the streets of Kiev I was surprised to hear Russian speech more frequently than Ukrainian speech.
As far as I know, the majority of Ukrainian-speaking population are in Western reqions of Ukraine. Lviv is the well-know example.