Devel.lulzbot.com is where the development files live. Download.lulzbot.com is where the production files live. The development files are named after things as codenames. Taz tend to be named after trees in alphabetical order. The latest taz 6 prototype is the olive folder. Inside the various folders you will see production parts, design files, documents, and production gcodes. In some cases you will see ao- 101 or mini subfolders. This may indicate that a particular batch of parts was sliced to be produced on an ao-101 or a mini printer instead of a taz. Sometimes smaller precision part batches benefit from less time on the heated bed (extruder gears for example).
A folder may also have different versions of a stl file. Usually that indicates one version is a newer, more refined revision. Sometimes there are stl files containing more than one copy of a part to make batch slicing faster (if you wanted to print a whole bed worth of idler latches for example)
A gcode file is a pre sliced stl file in printer specific format. I could slice a stl for my modified taz that someone who had a mini would not be able to print due to the bed size and speeds. But I could come back and load that same gcode file on my printer and hit print without waiting to slice it again. Generally you want to just slice the stl file unless you know where the gcode came from and that it is for your printer configuration.