Tears, pulls and strains all describe muscle damage -- they're more or less the same injury. They can result from muscles that haven't been stretched or warmed up properly, overuse of the muscle by a repetitive motion, or a sudden, powerful motion (such as jumping or even exploding out of the starting blocks in sprints).
A muscle injury shows up as a sudden pain in the middle of the run that doesn't go away and keeps you from using that muscle effectively. Later, it will become swollen, painful and possibly discolored.
The treatment is similar to many runners' injuries: rest, ice, compression, elevation -- a regimen often known as R.I.C.E. It is important to rehab damaged muscles with light exercise, heat and deep muscle massage to prevent scar tissue formation within the muscle [source: Anderson].