Grips should generally know about 5 knots. Bowline, Clove Hitch w/a Half Hitch safety, Trucker's Hitch and the one you tie your shoes with ( I thought it was called a Granny knot, Bzz, wrong answer, anyone know?).
Knots are an important part of work. Any Knots you don't know, you should pick up some sash or trickline and practice and practice. Then close your eyes and tie it, then tie it under a chair where you can't see it.Then get in your shower and turn it on as cold as it gets, lean out, arms around your shower curtain and tie it to the toilet. Sound Crazy? Well one day sooner or later, you will be on a roof in a winter rain storm, leaning over the edge, tieing off a 12x12 to a beefy point. If not tied correctly a Knot is ''deadly''. First Rule of Knots: You never leave one that is not tied correctly or if you are unsure, untie and do it again. If not tied properly it could fail and lose it's load or inconvenience the next Grip that has to untie your mess. Best to do it right. A Boy Scout is required to know more knots than the average Set Grip. If you are ever unsure of what Knot to use or how to tie it properly: Ask The Best Boy, always a good rule when in doubt. A great site for Knots is Grog's Animated Knots.

Bowline

Clove Hitch

Half Hitch

I generally only use a half hitch as a "Safety" (tie Half Hitch with loose end to keep it from untying), never as a bearing knot.

Shoestring Knot

Truckers Hitch

Bonus:

Water Knot

Don't walk away from a knot incorrectly tied. Anything involving ropes has the possibility of being dangerous and life threatening. Be careful and always do the safe thing.

Sorry for the mismatch of pics, I "borrowed" these from other sites until I take some.