There is a large variety of knots, each with properties that make it suitable for a range of tasks.
Some knots are used to attach the rope to other objects such as another rope, cleat, ring, or stake. Some knots are used to bind or constrict objects. Decorative knots usually bind to themselves to produce attractive patterns.
While some people can look at diagrams or photos and tie the illustrated knots, others learn best by watching how a knot is tied.
Knot tying skills are often transmitted by sailors, scouts, climbers, cavers, arborists, rescue professionals, fishermen, linemen and surgeons.
Over at animatedknots.com they have some extremely helpful animated “how to’s” and even more helpful videos for each of the knots to get you up to speed with your knot tying skills.