[ArtMonkDrillz]

Authentics are thicker and have the actual stitched on letters like the players actually wear during the games.
The replicas, which are all I've ever bought, are thinner and have the numbers 'painted' on (for lack of a better term). They're not bad, I think most people probably have replicas, especially since the players come and go so often.
There are also "repla-thentics," which are like a mix between the two. Their okay I guess. They're thinner than the real thing, but the letters are stitched on.

If you go through eBay, just really look closely at the picture and at other buyer's comments. My neighbor's bought my mom a Riggin's jersey for her birthday a few years ago and I almost cracked up when she unwrapped it because it looked so bad. People make crappy knockoffs at home and sell them online. Her's is practically purple and the stripes on the sleeves are all wrong.