Even in a year when the presidency isn't being decided, election day in the US is massive. There is voting for state legislatures, local councils, referendum initiatives, and more. But most of the interest is in three things: the two houses of Congress, and state governors. Let's take them in turn.
House of Representatives
The whole of the lower house is elected every two years. In 2016, the Republicans won a 47-seat majority, 241 to 194. The Democrats picked up one seat in a byelection (Americans call them "special elections") earlier this year, so they need another 23 to take control.