This tutorial Miles and I made show how to bevel certain sections of a polygon (as opposed to the whole object), being useful for changing the curvature and smoothing out polygons.
Bevel:
Creating a slope from a straight edge, the bevel tool is very useful for giving your objects rounded edges. However if you just use the bevel on an entire object, it’ll change the shape completely as it’s been selected to affect the entirety of the object.
Edge loop tool:
The edge loop tool can create custom edges across an object, segmenting it and allowing you to choose specific areas (increasing the subdivisions can also do this how the segments are spread evenly, whilst the loop edged tool allows you to select where to apply).
Jonathan's University Blog 