Okay, here's a little visual guide that explains it better than plain English, and I've included a few brands that I can see confusing to a newb. . . Selvage (RRL):

NOT selvage (Diesel):

^^Tightly sewn-over edges in bright thread; pretty, but no dice. Selvage (Nudie):

NOT selvage (Earnest Sewn):

^^notice the dark stitching sewing the edge contrasted by the orange stitching down the center of the edges, from afar it can be deceiving

Selvage (5EP):

^^here's one brand that doesn't use the common white selvage, but notice the fabric edges are continuous from the denim NOT selvage (Volcom):

^^kinda like Deisel Selvage (Paul Smith Red Ear):

^^another brand that uses non-white selvage, in fact, as you can see, they use some motley colored material for the selvage edges. By now, the first poster will hopefully not have any lingering doubts.