This is what a bioball looks like. If you can imagine a box full of these things, you can see all the area there is for bacteria to grow.
Here are two pictures of commercially available sump with an integrated trickle filter. These are the same piece, but one picture shows the bioballs in it and the other doesn't.
The water flows from the aquarium onto the drip plate with all the holes in it. From here the water trickles through all the bioballs, which rest on the black grid (which is called eggcrate). The middle compartment holds a sponge filter, and then the pump sits on the right.
This is a decent design. The water level is around the top of the sponge, which means only the bottom layers of bioballs are underwater (remember they work better when they're not submerged). Its generally difficult to integrate a wet/dry filter into a sump because you need to have a low water level. Its hard to run a skimmer or do anything else (like a refugium) with a really low water level.
The other alternative for a wet/dry is to put it before the sump, as I have done.
Here are two very good threads (Of course they're good, I started them

). The first is my education in filtration as I was asking questions as I planned my tank:
Tank renovation - Filtration
The next thread is photos of my finished setup.
Tank photos - Finally!
You'll learn a lot from these threads. They should be required reading for ceph-keeping; if I do say so myself
Dan