Basic Knots
These are the basic knots taught to all Boy Scouts. Know them and love them. Eventually you're going to impress your friends when you fix their tent when you're all camping together or when you help tie down that Christmas Tree to the top of their car.

Square Knot
 Animation of tying a Square knot The next knot you probably learned is the Square Knot. It's primarily used to join two ropes or to bind a single rope around an object. The problem with a square knot is that it can fail really easily.  It's a good basic knot, but if you need something that's secure then the better bet is a Sheet Bend.  

The Square Knot is also referred to as the Reef Knot.
 Boy Scout Handbook pg 21

Two Half-Hitches
 Animation of tying a Two half-hitches Two Half-Hitches are usually used to secure a rope around a cylindrical object like a tree or pole, but that's not it's only use.  Two Half-Hitches is has a variety of uses and can be used to tie a rope to a variety of different objects. This knot is only secure when it's snug up against the object you've tied to.  Boy Scout Handbook pg 384

Taut-line Hitch
 Image:Tautline Hitch.gif The Taut-line Hitch is the knot that's going to save your bacon when the guyline on your tent's rainfly just busted.  The Taut-line allows you to tighten or loosen a rope that is tied to an object. Knowing this knot is essential for camping.

The Taut-line Hitch is also know as the Rolling Hitch.
 Boy Scout Handbook pg 385

 Animation of tying a Bowline The Bowline is probably about the most useful knot you can learn. It forms a secure loop at the end of a rope that won't tighten and it's easy to untie once the load is removed from the rope. Be careful when using this knot because when it's not under load it can come untied easily.  Boy Scout Handbook pg 388

Sheet Bend
 Animation of tying a Sheet bend  The Sheet Bend is used to join two ropes together. It's more secure than the Square Knot and unties easily when done.    Boy Scout Handbook pg 390

Clove Hitch
 Animation of tying a Clove hitch The Clove Hitch is used to secure a line to a post or other cylindrical object. It can also be used as the start of lashings. The Clove Hitch suffers from a problem with slipping down if it's not held under tension and if it's exposed to extreme loads it can bind and be difficult to untie.  Boy Scout Handbook pg 387

Timber Hitch
 Animation of tying a Timber hitch  The Timber Hitch is used to attach rope to a piece of wood.  It's the knot you'll need to tie a rope to that log that you're about to drag back to camp for firewood.  According to the Boy Scout Handbook this is the knot that should start your diagonal lashings.  Boy Scout Handbook pg 386