iv heard people saying they are join 21b and stuff like that. Is there a list somewhere showing what they all are?

Army Military Occupational Specialties (MOS) can be looked up at http://www.GoArmy.com

21B is Combat Engineer. This is an example of how the Army contracts recruits into a single specific job provided they are mentally, morally, and physically qualified.
The two numbers denote Career Management Field(CMF) and the letter identifies the specific specialty. And the numbers and letters following show skill level, and special skills, etc. My MOS for example, 19D34D3, 19(Armor) D(Cavalry Scout) 34(Skill Level 3{30} Army Recruiter {04}) D3(Bradley Fighting Vehicle/HMMWV Crew).

These only apply to the Army, as Marines use a numerical series, the Navy have Rates, and the Air Force uses AFSC.