Perrformance T-shirts are made for sports, they feature taped seams and no labels to rub or irritate your skin, and have wicking features which take sweat away from your skin and help cool your body.  They stretch well to allow movement, and do not hold as much moisture when wet which makes for more comfortable exercise.  Unlike traditional cotton t-shirts they do not show sweat as much, dry fast and retain their shape when wet.