What's are the differences between a pipe wrench, a strap wrench and a monkey wrench?

A pipe wrench and a monkey wrench are essentially the same thing.

A strap wrench uses a strap of fabric, steel chain, or other flexible material rather than adjustable jaws to grip an object. Strap wrenches have an advantage in tight spaces and are less likely to mar the surface of the work.

A pipe wrench, often called a Stillson wrench resembles a monkey wrench but with serrated jaws, one of which is capable of slight angular motion so that the grip is increased by pressure on the handle. The original monkey wrench is probably an antique and was an all around tool with heavy jaws for tightening bolts and nuts or hammering on things.