There are three major identifying variations of the PS2 console, as represented by their physical form-factors. Within each form-factor, one can expect to final additional variations based on usable hardware features or manufacturing tweaks that are invisible to most people.
The First PS2 (Fat PS2) Starting with model SCPH-10000 Hardware Features: FireWire port, Two USB ports, Rear Expansion Port, Large Form Factor
The second ps2 version (Slim PS2 Or PSTwo) With minor hardware changes/upgrades: No FireWire, No Expansion Port Hardware Features: 2 Front USB ports, Built-In Ethernet Network port, Small Form Factor
A rare Japaneese white PS2/DVR combo with a built-in TV tuner and hard drive for recording TV shows. It also played standard Playstation 2 games and used the Sony XMB interface similar to the PSP and Playstation 3 Hardware Features: 4 USB Ports (2 Front 2 Back), TV Tuner, DVR Capability, Hard Drive, Large DVR Like Form Factor