Roughly speaking, about four out of every six people in Ukraine are ethnic Ukrainian and speak the Ukrainian language. Another one in six is ethnic Russian and speaks Russian. The last one-in-six is ethnic Ukrainian but speaks Russian. This map shows where each of those three major groups tend to live.
Here's why this matters for what's happening in Ukraine now: Since it declared independence in 1991, the country has been politically divided along these ethnic-linguistic lines. In national elections, people from districts dominated by that majority group (Ukrainian-speakers who are ethnically Ukrainian) tend to vote for one candidate. And people from districts with lots of ethnic Russians or Russian-speakers tend to vote for the other candidate.
