Surrealism is a broad term generally used in the arts world, to describe something that is bizarre in an almost dreamlike way; where each individual element of the scene looks right, but joined together they produce an impossible juxtaposition that confuses the mind. Surrealism is most commonly associated with the artist Salvador Dali.
Surrealism also creeps into the realm of humour occasionally, and there are a handful of surreal jokes on BadPuns.com.
I went fishing last week. I used a dotted line.
In this case, a dotted line is a concept we're all familiar with, as is fishing, but combining the two concepts produces a surreal result.
Much of the humour considered to be surreal actually falls into the category of non sequiturs (qf), though the two are closely related.