To win the presidency, a candidate must win 270 electoral votes. To win electoral votes, a candidate must get a relative majority of votes in a state. Each state is worth a different number of electoral votes (roughly proportional to the population of the state). Based on current polling Clinton will win 323 electoral votes and Trump will win 215.
The majority of states are either safely Democratic or Republican and as partisan ties become stronger some states that traditionally swing have stayed with one party for the past few elections (for example, Virginia for the Democrats and Georgia for the Republicans). Trump's rejection of the establishment GOP and alienation of hispanics and other ethnic minorities have put traditionally Republican states into less secure positions (like Texas and Alaska).
Maine and Nebraska have 2 electoral votes statewide and 1 per district. They each have one electoral vote that is a swing vote.