Its a race, to build your base! Steal Construction Materials from your enemy's base and secure them at the various 'Capture Pads' located in your team's base. Different capture pads correspond to different base constructions and upgrades, so be sure to think about where you secure your Construction Materials. Capture Pads can be used multiple times to upgrade constructions. Also, constructions can be 'synergized' by completing combinations of other constructions. The first team to complete all 40 constructions or have the most completed when the timer runs out wins!
That's the basics of the Baserace maps by CptnTriscuit, in which the aim is to construct your base using stolen enemy parts, while at the same time stopping the enemy from doing the opposite. Understandably, the map itself can become slightly complicated, not to mention the unfamiliar style of CTF (capture the flag) gamestyle in ET.
Because of this, CptnTriscuit has written a quite brilliant and extensive guide to all the maps, not just for the latest release B3, which should help some of you players new to the maps out there. Here is the layout to the guide: