I have no clue-I am a recreational player who wants to join FIDE tournaments.
Rapid and Blitz are both form of fast chess
Rapid, as the name suggests, is a faster version of chess
Time controls for each player in a game of rapid chess are, according to FIDE, more than 10 minutes, but less than 60 minutes. Rapid chess can be played with or without time increments for each move.
Many tournaments with a large player count opt for this faster format to try and finish off an entire tournament in a day or two.
Blitz is even faster than rapid, with times less then 10 minutes on the clock.
Time controls for each player in a game of blitz chess are, according to FIDE, 10 minutes or less per player. This can be played with or without an increment or delay per move—a more recent development due to the influx of digital clocks.
Both these formats have their own rating category in FIDE in 15+10 and 3+2 time control respectively.
They are time controls for chess used in relatively quick short games.
The actual time amount depends on how is defining them.
Blitz used to be zero seconds per move before digital clocks evolved. Rapid used to mean exactly ten seconds a move before clocks came into more widespread use and then it was 5 minutes (or ten minutes) per person for the entire game. EG SD5 or SD10.
All were popular speeds back in the day.