Originally posted here by Arcanus Aegis
Another addition:

UDP, being broadcasted, does not pass through routers since routers do not pass broadcast traffic. This is why UDP will only be found small, internal networks, and TCP, even with it's slightly greater overhead, is much more common.
UDP will only be found on small, internal networks? Man you just made my jaw drop. TCP is more common because it is connection-oriented and provides "reliable" transport of the data. Routers don't pass any broadcast traffic regardless of if it's TCP, UDP, ICMP or anything else. That's why they are said to create Broadcast Domains. However UDP does not have to be broadcast, you can send a udp packet from one computer to another. A good example would be the original Back Orifice, it ran on udp port 31337. People used it over the internet all the time. Routers didn't drop it, and last time I checked the internet wasn't a small, internal network. Anyways.. thanks for the laugh.