Although contemporary ties come in all sorts of styles, there are relatively few knots in common use today. This is a far cry from Beau Brummell's day, when fashion manuals illustrated 32 ways to tie a cravat. In addition, gentlemen would often improvise their own knots.
The four-in-hand knot is the virtually standard in the United States. The more complex windsor (invented by the Duke of Windsor), and the half Windsor, are more popular in Europe and South America.
The knot should not be so large as to spread out the collar, nor should it be so tiny that it can hardly be seen.