Evo,
TCP is a connection-orientated protocol, UDP is connectionless.
Basically this means for TCP , every lump of data that is sent has to be acknowledged, there is flow control, guaranteed delivery of data etc. but more overhead as a result.
For UDP, also known as 'say-it and spray-it, data delivery is on a best effort basis, i.e. no explicit acknowledgement of data, no explicit flow control etc. but less overhead.
Examples of TCP : SMTP (Email), Telnet (Remote login), FTP (File Transfer)
Examples of UDP : TFTP (basic file transfer), SNMP (Network Management), DNS (name resolution).
So, TCP is used for Email as it's important that delivery is guaranteed.
UDP is used for DNS, so say when you look for
www.pprune.org and DNS goes off to find the IP address, it's less important if the data is lost as your browser will just have another go.
It's a tad more complex than that but I was trying to keep it understandable !
Cheers