I'll describe what Seiberg-Witten invariants of 4-manifolds are, recall several theorems which are used to compute them, and give examples of simply connected 4 manifolds with small 2nd homology which admit infinitely many smooth structures.
Links:
[1] http://www.mpim-bonn.mpg.de/taxonomy/term/39
[2] http://www.mpim-bonn.mpg.de/node/3444
[3] http://www.mpim-bonn.mpg.de/node/2761