The GO models are the high end models, they offer added features like an SD slot, bluetooth handsfree calling, more customization, etc.
The TM is a flavor of a product line (eg: there's a GO 740 TM and XL 350 TM, among many others). TM means it has lifetime FM traffic and lifetime maps included with the purchase.
LIVE is a added feature currently only on two models in the USA (XL340S LIVE and GO 740 LIVE). LIVE models have an internal modem and allow the device to use a number of Tomtom data subscription services, including traffic, fuel prices, buddies, weather, and speedtrap alerts. The traffic bundled with LIVE is a class above the FM traffic on other models, so it is a better option if traffic is important to you.