Records Explained
Despite knowing a ton of technical information about how vinyl records are made and played, I still think that they are partly magic. To put it simply, the groove on a record is one long soundwave that's been stamped into plastic, and it can be read and re-played by dragging a needle/stylus across it. This still blows my mind, and more so when I think of the records I have that are older than my parents that still play perfectly and sound great.
Sizes:
7" - mostly singles, usually one track on each side.
10" - mostly EPs (Extended Plays) with 2-3 tracks on each side.
12" - mostly LPs (Long Plays) or 'Albums' with 4-6 tracks on each side.
Speeds
33 1/3rpm - standard speed for the majority of12" LPs (Albums). Some 7" singles use 33rpm to have longer time per side.
45rpm - standard for 7" singles. Some modern 12" records are 45rpm
78rpm - legacy format for very old pre-plastic/vinyl 'shellac' records. These require a separate stylus / needle
Higher speeds generally mean more fidelity (think about it like bitrate), but the compromise is that you're going to move through the disc-space faster. I have a couple of 12" 45rpm albums and they sound fantastic.
Loudness - the louder a physical soundwave is the bigger and wider the physical groove has to be. It also means that if an album is mastered quietly, it can contain over an hour of music - far beyond the 40-50 minute 'limit'.
Due to some complicated physics the needle on the outside of a record can handle bigger/louder songs than when it's tracking the inner edge of the disc. This lead to a load of classic albums having a quiet song at the end of each side.