difference between inbound and outbound traffic

I fininshed setting up cacti graphs for our F5s. The Traffic graphs I created for each interface (from a template), have an area for Inbound and a line for Outbound. The Inbound number is always higher than the Outbound number.

A sample is attached to this post (I think) or here http://i76.photobucket.com/albums/j...raffic.png

The Inbound and Outbound numbers are non-zero, but they never vary. What are they referring to? Is tracking them good for anything? If you've got a pointer to documentation I'll take it--haven't been able to find anything myself.

Thanks.