Differences in distances between hands, hand-angles, & body-angles (relative to the floor or whatnot) make for different effects. There's a thread around here (or the body-building section) about chest exercises -- look for my post & note some of the body-angles for push-ups.
* Generally, the closer your hands, the more your triceps are affected, & the further-apart your hands, the more your chest is effected.
* Generally, the higher your feet relative to your hands, the more your upper-chest is worked, & the higher your chest relative to your feet (in the down position) the more your lower-chest is worked.
Also both shoulders & triceps get some work... when chest is higher than feet (in down position), chest gets worked less & triceps more if/when the feet are moved forward... so do parallel-bar dips with feet back, chest-forward to work chest best.