ok, well there are TONS of differences between a replica and a real designer bag.

-for one most replicas are made in sweatshops,and then illegally imported into the U.S, so if you're thinking about buying one, dont.

-however if you just want to know the difference then:

-the real thing is ALWAYS better quality than a knockoff.  im sorry, but i think if you pay hundreds or thousands for a bag, you're going to get better quality then if you pay 20 bucks

-its easy to tell a knockoff from an authentic bag in most cases because:
         
1. the leather is fake.  almost all of  those quilted
             "leather" bags you see are fake leather, and you can   
              tell because the leather is way too reflective, and   
              looks plastic.
       
  2.  A real designer bag will actually have the designers
              logo on it somewhere.  If you look at the knockoff
              Coach's you can tell that the C's are actually G's or
              they are connected, so that it looks like the bag is
              real when its not.
         
3.  It's not hard to tell a knockoff from a authentic bag.
              everything on a replica is of lower quality.  On a
              replica the lining is fake silk, the leather is not real,
              and nowhere on the bag will it actually say the
              designers name.

I hope this helps, and remember if someone wants you to buy a knockoff "Just say no."