Well in B/W there are 2 types of GTS:GTS Negotiations and the normal GTS.
The normal GTS works like it did in D/P/Pt and HG/SS. You can either put a Pokemon up for trade and ask for another Pokemon in return, or you can go look for someone trading the Pokemon you want. Note that the Pokemon you ask for or the Pokemon you look for has to be in your PokeDex.
GTS Negotiations is a new type of GTS introduced in B/W. It works differently from the one mentioned above. You look for someone to trade with and when you do find someone it works like trading normally, only you cant talk to the other person. The yellow happy face means you want the Pokemon, the the blue sad face means you don't want the Pokemon. When you both find the Pokemon you want you can trade it.
Source: Personal experience.