3 sided polygon
 

Why is some models created with 3 sided polys and some 4 sided?

It depends on how the models will be used.

Many games require 3-sided polygons in the meshes, like UT2004. Other uses aren't as picky. In general, it's easier to work on a mesh that can have polygons with more than 3 sides.

When modelling for video games the mesh needs to converted to tris. I believe you can judel model as you usually would, then get Maya to convert to tris.