Like, underneath your name, it will say "Member", "Junior Member", "Senior Member". Or something random that the person made with ToriPrime. What changes it and why does it change? Mine is just "Member".

It's a default user title that relates to the amount of posts you have. You can also buy a custom one here: http://forum.toribash.com/tori_usertitle.php

0 - 29 Posts - Junior Member
30 - 499 Posts - Member
500 - 1499 Posts - Senior Member
500 - 2499 Posts - Forumite
2500 - 3499 Posts - ub3r
3500 - 4499 Posts - Raconteur
4500 - 5999 Posts - Chatterbox
6000 - 9000 Posts - Elitist
9001+ Posts - Flagrant Meme

as NotShadow said, also here are all the non-custom user titles and their conditions.

JtanK Moderated Message:

To sum up: posts, panel in torishop, changing in user cp when you have prime/vip, paying $ in nabi customization store (stickied thread in items board of market).