Can also be used to express understanding of a subject, theory or plan.
The word is usually yelled obnoxiously in a manner such as lil jon would say yeah!
Derived from the name of the some-what popular go-kart brand Yerf-dog.
Also may be used anytime you make a powerful/sharp turn in a car.
Another use for the word is to yell it when looking for a friend in a crowd or in the woods, in this case yerf can be exchanged with the rapper "birdman's" signature call BRRRRRRRRRRR (as to imitate a pigeon or other bird-like creature's call)
Guy 1: Dude lets go smoke then get some slushies at Wawa
Guy 2: YEEERRRRRRRRRFFFFFFFFF!!!!
Guy 1 pulls into guy 2's driveway quickly and smoothly and lets out a mighty "yerf" to signify his entrance.