A cleat is a fitting on a boat that is used for attaching a line. There are several different types with different uses. Most cleats are designed to be used with a specific size of line and may not work well with the wrong size.
A horn cleat (or mooring cleat) has two horns. A line is cleated by wrapping the line around the cleat once, then crossing over the center diagonally, then secured by by putting a loop in the line that places the bitter end of the line under the standing end. The initial contact with the cleat should be either parallel to the cleat, or at an acute angle to it.
A cam cleat uses two spring-loaded cams to grip the line. A line can usually be secured to a cam cleat by simply pulling it through the two cams. The line is released by pulling it up out of the cams.
A jam cleat has no moving parts but works by jamming the line in place by virtue of its shape. It incorporates a wedge shape to allow a line to be pulled easily in one direction, but it will grab hold of the line if it's pulled in the other direction. Much like a cam cleat, the line is release by pulling it up out of the cleat.