From google, so sounds like it's the least serious type
A Hamstring injury is common in sports. Hamstring injuries are sometimes known as a 'pulled Hamstring'. The term 'pulled muscle' comes from the description of how the injury takes place. Usually, the Hamstring muscle is forcibly stretched beyond its limits and the muscle tissue becomes torn. A tear in the Hamstring muscle is referred to as a Hamstring strain by medical professionals and depending on its severity it is classified as a first, second or third degree strain:
a first degree strain is damage to a few muscle fibres,
a second degree strain is damage to a more extensive number of muscle fibres,
a third degree strain is a complete rupture of the muscle itself.