scroll for a good diagram: here
wet/dry = place in sump where typically biomedia/bioballs are placed and water trickles over them and they are exposed to air and water. so think of a box with two grids or egg crates and in-between them bioballs . water is sprayed or injected on the top and it trickles down the bioballs and collects in the bottom reservoir where it then is pumped back to the tank. this creates an area where beneficial bacteria grow like crazy with oxygen being available all the time. hence the name wet/dry.
a sump is a plastic or glass box under the tank that has baffles to regulate the water in the sump. so you have 3 baffles, that is three chambers. assuming left to right:
1. Water Intake, this is where water enters the sump through a micron filter or sump bag
2. 2nd chamber can be used as the wet/dry section or in saltwater where the skimmer goes, heaters and other misc stuff as well can be placed
3. return pump chamber, this is where the return pump is placed.
That is the basic sump design, most sumps only have two chambers though, the first being combined with the second. Of course there are loads of variations to suit the application intended, for example in saltwater, a 5 or more chamber sump is not uncommon, namely, intake, skimmer section, refugium section, frag section, return pump section and ATO reservoir.
hope that helps