The term ‘knots’ covers several different types of line tying. There are hitches, where you tie a line to something else; there are bends, where you tie one line to another; and there are knots, where the line is tied to itself. The aim of them all is to secure your boat when you want, but allow you to untie them when it's time to push off the dock again.
You don’t have to be an old salty seadog to master the essentials. Here are three basics that are easy to learn and will serve you well. For more details and some great photos, read Alex Smith's post on boats.com UK site, Essential Power Boating Knots.
Make a loop in the line. Feed the end up through the loop, then around the lower part of the line and back through the loop. Pull the ends tight and test to make sure there is no give.
2. Clove Hitch
Go around the object once, make a second turn around in the same direction crossing the first, and then tuck the free end through the eye of the second turn.
3. Round Turn with 2 Half Hitches
Go around the object twice (more for a large load). Then tie a clove hitch around the line itself.
For more knot tying info, watch How to Tie a Line on a Cleat. And if you're new to boating, you might want to watch How to Dock a Powerboat.