Arusha is a crowded little African town, full of life. A hub for many tour operators around Serengeti and Mt. Kilamanjaro.
Arusha is primarily a transit point to nearby attractions but the city is also a great place to relax for a day or two before / after an adventure elsewhere.
There is not much to see in Arusha, but most tourists start their trip here. I'm not sure how safe it is to just stroll on the streets without a local guide but I wouldn't try it myself. The local Airport looks like a small bus station and it's hard to understand that this is the main landing point to all the tourists that come visit the area.
Nothing realy special but just a nice place, good bars and of course start point for trips to Serengeti and Ngorongoro