Hey Max!
The difference between Quads and Tris is as the name suggests, Quads have four vertices/edges, tris have only three.
Quads are preferred over Tris or Ngons (a Face with Five or more vertices/edges) because of many tools, modifiers and textures can have adverse effects with Tris and Ngons.
For example, the Loop Cut tool (CTRL+R) doesn’t cut through Tris or Ngons, Tris with a subsurf modifier can cause pinching.
Quads and Tris are not completely forbidden, however. They can still be used for speed and convenience. It all depends on what and how you are going to use the model.
Hope this helps!
