How do elections work?
The 435 seats in the lower house of the federal congress, the House of Representatives, are re-elected every two years. (There is no term limit and incumbents are often re-elected, so there is substantial continuity among members). Seats in the House are largely proportional to population, so California has a lot of seats but Montana only a few.
The upper house, the Senate, has 100 seats; two for each state, no matter how populous. Every even-numbered year, about one-third of senators are elected for a six-year term. Legislation must pass through both chambers to become law. Senators have additional responsibilities compared to their colleagues in the Chamber; it is mainly about confirming (or not) the presidential nominations.
Governors are not part of Congress. Their impact is primarily local within their own state, but because they may affect election law or practice, they may affect future federal elections.
Prior to the election, President Joe Biden’s Democratic party controlled both houses of Congress, but with very narrow margins. In the Senate, a 50-50 split would mean a casting vote would go to Vice President Kamala Harris. (In practice, the casting vote went more often to Joe Manchin, a Democratic senator representing a largely Republican state).
These elections are called midterm because they take place in the middle of a four-year presidential term. They are difficult for sitting presidents because the presidential party often does poorly in them and because losing control of Congress makes it harder for the president to pursue his agenda.
About these results
The results are provided by The Associated Press, or AP, which has rigorous criteria for “calling” electoral races; that is, to signal a winner. The reshuffling of House districts this year following the 2020 census means the AP is not reporting “turnarounds,” or seats where the sitting party changes. Redistricting also means that in a few seats there are two incumbents competing for a new district. It is also possible for some seats to have 100% of the votes without a winner emerging due to laws that trigger an automatic recount in very close races.