UDP and http are on totally different layers of networking. UDP is a stateless version of TCP. Being stateless, it is less complex and less "heavy" than tcp. It also make udp less reilient against missing or corrupt packets. But udp trackers can handle many more peers at the same time than a tcp tracker.
Http is a application level protocol and is populary used in www to specfy how a client communicates with a server.
Seeing as your query looks like the one in the link above, http tracker protocol vs udp tracker protocol is what yu are after. Normally, the http dialog is carried over tcp/udp to the server/client. With udp traker, they seem to have eliminated http and use only udp for dialog. They claim 50% increase in speed in communication with trackers over the http technique. So if your client supports udp tracker urls, then you can choose udp.
For more detail, you may want to wiki or google.