The elections are typically bundled, so in a particular November in some places you might be voting for president, senator, representative, governor (most states have gubernatorial elections halfway through presidential terms, though), state representative, state senator, district attorney, sheriff, school board, propositions (binding referenda on legislation or amendments to state constitutions), ... Turnout varies a lot, with much higher turnouts in presidential elections (but these additional voters may or may not vote on the other ballots).
In the federal govt AIUI there are no elections for executive offices other than the presidency, so only president, vice-president, senators and representatives are elected. But many state and local executives are elected, as determined by local legislation.
http://images.dailykos.com/images/134822/large/Elected_offic... gives a breakdown of how many elected officials there are at various levels of government.