A bulging or pushing rise, where the trout's feeding causes a bulge in the surface, indicates that the fish is feeding on emerging nymphs just under the surface. Also if you see a dorsal fin and tail it indicates sub-surface feeding.
A boil is a bulging rise where the surface gets broken slightly, this usually indicates that the trout are taking larger insects, such as Pale Morning Duns, just under the surface.
When you see the fishes snout poke out you know they are feeding on insects in the film or on the surface. An air bubble left on the water is a sure sign of a surface feeding trout.
Splashy rises can indicate a few different things. A small fish tends to make more surface disturbance than do larger ones. The bigger fish will expend as little energy as possible when feeding, especially on smaller flies. Whitefish will also make splashier rises than do trout.
The other time trout make large splashes is when they chase emerging caddis flies to the surface. They are not really feeding on the surface and the "rise" anglers are seeing is actually just the momentum the trout develops chasing the caddis up from the bottom propelling it above the water. This "rise" is best fished by letting a caddis pupa pattern sink and then swinging it on a tight line, the strikes on this type of technique are quite fierce.