Mostly it's about quality of the fabric. Authentic shirts are made out of durable fabric, the patches are sewn on and letters and numbers are not attached, but made as a part of the shirt itself (not completely sure on this one), so they don't wear off that quickly. Replica shirts are made out of worse fabric, patches are mostly ironed on and letters and numbers are attached with some kind of glue. Basically, if you have a lot of money and want the best quality = go for the authentic. If you just want to have a nice looking Juve shirts that you can use occasionally - go for the replica.