Worf was born into the house of Mogh on the Klingon homeworld. When he was six, Worf's parents were killed in an attack by Romulans. A Federation Starship answered the Klingon's distress call and Worf was adopted by a Russian couple who raised him.
Worf became the first Klingon in Starfleet and was eventually posted on the U.S.S. Enterprise-D where we meet him in Star Trek: The Next Generation. He is security chief for most of the show. Later in his career, while taking an extended leave from Starfleet, Worf was called to Deep Space Nine to act as a liaison between Starfleet, the Klingons, and the Cardassians, whom the Klingons had attacked. Eventually, he stayed on Deep Space Nine.
And now, Ten Reasons why we love Worf.
10. For complimenting Major Kira.
Julian: ...and Major Kira Nerys, our first officer.
Worf: Nice hat.
Kira: I don't normally dress like this.
9. For (sort of) tolerating Lwaxanna Troi's mispronunciation of his name.
8. For teaching Wesley Crusher about Klingon courtship methods.
4. For helping Keiko O'Brien to give birth.
"Congratulations, you are fully dilated. You may now give birth."
3. Because instead of drinking Blood Wine like any other Klingon, Worf drinks prune juice. "It is a warrior's drink."
2. For killing Ole' Googly Eyes.
(No, that's not the moment of death. That's what Chancellor Gowron looks like normally.)
1. Because even though he's the biggest guy on the Enterprise bridge - and a fierce Klingon warrior to boot - he seems to be beat up by EVERYBODY (but especially Counselor Troi).
Bonus: For taking care of Data's cat.