Cirdan is an elf of the first age. Whether he was one of the Elves that first awoke at Cuiveinen or not is speculated, but it's clear he is one of the oldest elves (being told he has a beard, Tolkien said only the very oldest elves have beards).
He is the Lord of the Grey Havens (a port in western Middle-earth where elves go to sail west to the Undying Lands). He was the original possessor of one of the elven rings, then when Gandalf came to Middle-earth, he gave it to Gandalf.
He was involved with the wars against Morgoth in the earlier ages, as well as helping the Dunedain in their war against Angmar, but since then he's stayed in the Grey Havens and will remain there until the last elf leaves for the Undying Lands.
I hope that gives a nice, brief introduction.
