Hillary Clinton and Donald Trump are the candidates for president. Barack Obama's term is up in January of 2017. Every leap year is a presidential election year (plus century years like 1900 and 2100).
Elections for representatives happen every two years (including every presidential election year). Historically we haven't had national debates between legislative candidates (the Lincoln Douglas debates of 1858 are arguably an exception). The United States legislative candidates are elected geographically: by state for Senators and by congressional district for Representatives. Congressional districts are always within states. They do not cross state boundaries. A single state may have one or many congressional districts, depending on population. Every state has at least one.
To complete things, only one third of Senators are up for election every two years. Senators serve staggered, six-year terms.
There are 435 Representatives and 100 Senators (two per state).