<img src="http://ccl.northwestern.edu/netlogo/docs/images/intersecting-polygons.png" border="0" />
When whe talk about overlapping , we talk about multiple objects touching each others , as described on the picture above
There's 2 primary events when talking about collision
And the main difference between the two is that ...
On Collision only fires once , when the two collision boxes enter in collision
is Overlapping fires when the two collision boxes are overlapping
and will continue triggering until the two boxes aren't overlapping anymore ...
You could say that on collision is a Is overlapping with a trigger once but that's another subject <img src="smileys/smiley1.gif" border="0" align="middle" />
P.S: Read further on the manual and get a free badge ! ;)