This is an excellent question that everybody asks.
Steric strain occurs simply because two groups do not want to occupy the same space and the molecule must distort to accommodate that. Steric strain is the main strain involved in gauche interactions, for example in the gauche conformation of butane.
Torsional strain is more complicated: http://en.wikipedia.org/wiki/Strain_(chemistry)#Torsional_strain
Research suggests that torsional strain includes a steric component in some molecules, but also something more complicated. Staggered conformations are stabilized by a stereoelectronic effect called hyperconjugation, where a filled sigma bonding orbital donates into an unfilled sigma antibonding orbital. In the eclipsed conformation this stabilization is lost, and this lack of stabilization results in torsional strain.
All torsional strain includes a stereoelectronic component, but not all torsional strain includes a steric component. For example the torsional strain associated with the eclipsed conformation of ethane does not have steric strain because hydrogen atoms are too small. In contrast, the strain in the highest energy eclipsed conformation of butane (with a methyl-methyl eclipsed interaction) includes both a steric component as well as a stereoelectronic component.