Dixieangler has given you some good links to check out. The lower the fly line the smaller the fly that you can throw. So something along the line of a 2-4 weight line is ultra light fishing and is great for small stream fishing and gills. 4-6 is more of a general use for trout and smaller fish. 6-9 starts getting into bass fishing size lures such as the bigger hair poppers and streamers. 9+ you are getting into salt water size lures and are not much fun to throw all day. The action of the rod is described by the actions of it such as fast to slow action.
If in doubt, cross their eyes!!