From what I can see;

The top unit is a switch. This is basically where your network will live. Multiple computers will connect to the switch whereupon they can communicate with each other via layer2. It's similar to a home switch but much more versatile. It has 2 x 100baseFX (100Mbps fiber) uplink ports, one of which will connect to -

(The bottom unit is) a router. It's job is to provide layer 3 TCP/IP routing from your network to the internet. It also converts the physical media from ethernet to serial at the same time. The serial connection will connect to the T1 equipment.

This is where it gets a bit trickier, and where I'm starting to guess. The black and white boxes are both modems. I think that the black modem is being used to connect to your T1, and that the white modem might be there for Out Of Band access. (ISPs typcially provision OOB so that they can reconfigure equipment remotely.) It's not clear where the 2 grey Cat5 connections go to, but if they connect the white and black boxes together then this would make sense.