Well, ericr did a really nice explanation, and i probably wont beat that, but to try and help, ill take a whack at it:
Light tickling affects the skin more. For me its kind of tingly, and almost like a pleasurable itch (not an annoying one.) Run your fingernails down your arm, and imagine it intensified a lot (since you cannot tickle yourself.) It feels as if nerves become concentrated in the area being stroked, and causes you to tense up a bit, trying to pull back from the sensation (however, the sensation is VERY enjoyable.)
Harder tickling is more pressurized, and is more pinchy and poky. it hits pressure points, and if done right (not to hard or too soft) creates a butterflyish feeling in that spot. This tickling can be much more intense than light tickling in certain spots, like the back, ribcage, and hips.