Different variations of redirects, 301, 302, 303, 304 etc
Basically HTTP Status codes are used for computers to understand in what way they should talk to each other. It is like the computers way of making discretion between a snobby lady and a street kid with a baseball cap. It might still not work, but you are a lot better off if you request a service from them in two radically different ways.
On a more technical note, HTTM Satus codes tell the two computers chatting with each other in which way and from where the two should interpret, fetch and forward the information they are processing from eachother.