There's a picture below but I'll try to describe it as well.
Where two pieces of wood cross over and touch face to face. They are fastened with glue, nails, etc. One piece may overlap the other (see edges of pallet) or not (see corner of pallet). Note that the pieces are not cut - apart from to length.
What do you call this joint?
The answer is actually sort of in the wording of the Question, these are a type of overlap joint, sometimes called an overlay joint.
Because the pieces of wood don't interlock in any manner it makes construction very fast, but it's little used outside of rough carpentry. In a 'proper woodworking' context you might only see this used for the construction of shop furniture, or utility shelving.
Unfortunately these days the overlap joint is being mixed up with the lap joint (such as in the Wikipedia entry *sigh*) but, in woodworking at least, a lap joint is exclusively where one or both pieces have cut joinery and the two pieces don't merely lay on top of each other.
