UDP, TCP, UDP and TCP
Suppose u are looking at the video on you tube and quality of video is not HD then u are just using UDP.When u want to see video in 720HD,1080HD or 2160HD in that case TCP is used.
For file transfer we use TCP but for file downloading we use FTP or for small file we use TFTP.
DNS,RIP,OSPF,BOOTP,DHCP,NNP,NAT,TimeStamp and quote of day all uses UDP.
SMTP is application layer protocol but it uses TCP for sending mail.