A knot is any of several ways to tie a rope.
A bowline is knot used to form a fixed loop at the end of the line. It's easy to tie and untie and does not usually break under pressure. Therefore, it's been referred to as the King of Knots. Sailors are usually expected to be able to tie this knot blindfolded and to tie it behind their backs. In sailing it is often used to connect the sheet to the sail.
The Figure-Eight is another basic maritime knot. It is used to form a figure of eight at the end of a line. It is mostly used as a safety knot to stop a sheet from going entirely through a block.