There is a lot of variation in both lump and briquet charcoals. The most basic differance is lump is generaly charcolized wood that has not been reshaped and put back together with binders into the shape of a briquet. Most good quality lump will still look like pieces of tree branch after a forest fire. But there is also a lot of lump made from lumber mill hardwood trimmings as well.
Briquets are basically the charcoalized wood ground and reshaped with adders and binders to make a dense uniform shape. Some briquets are more natural than others - meaning they have less fillers. Usually best ones you can find will just have cornstarch added as a binder and thats it.
Performance differances: Lump burns hotter, and generally gives a bit of flavor smoke as well. Briquets burn more consistant (less temp. spikes), but less flavor and generally more ash.
Here is a great link that reviews lump charcoal:http://www.nakedwhiz.com/lumpindexpage.htm?bag
After much trial and error I found I like to use a mix of about 50-50 or 60-40 Lump to Briquets. I get nice long burn times, good heat controll, some flavor, and not as much ash as all briquet. I also toss on fist sized chunks of my flavor wood (hickory, mesquite, oak, ect.)