The trefoil knot is a famous knot. One of the reasons is that in the list of knots, ordered by crossing number, it is the first ‘real’ knot one meets, being the simplest non-trivial knot. (The first knot listed is usually the ‘unknot’, i.e. the unknotted circle.) The trefoil has crossing number 3.
Here is a traditional view:
Here is a depiction with bridge number 2:
The knot group of the trefoil knot (calculated either by the Dehn or Wirtinger presentations) has two very useful presentations: