What is the difference between presidential and congressional elections?

Congressional elections are every two years. Every other congressional election is also a presidential election. Two of every three elections you also elect a senator from your state. (Senators serve for 6 years and their elections are staggered so you usually don't vote for both senators in one election).

Those elections that are -just- congressional tend not to get as big a voter turnout. Most people don't even know who their congressional representatives are. More people care about presidential elections.

Congressmen and Senators are elected by direct vote--whoever gets the most votes win. Presidents are elected by the electoral college, so whoever gets the most votes in your state gets -all- your state's electoral votes.