That's a PowerPC-based mini, which was sold between January 2005 and Feb 2006. It can use a maximum of 1GB of memory and the latest operating system it can run is OS X Leopard 10.5.8. Depending on the model, it can have between a 40GB and 80GB hard drive in it. Specs can be found here. Note the Apple specs don't include the last "silent upgrade" to a faster processor, those specs are here.
In terms of what you can do with it, as long as you don't want to view streaming video's from the internet or view web sites that use the latest techniques, there's quite a bit you can use the mini for. It would handle email fine, as one example. If I was going to use it, I'd be sure it had 1GB of memory and probably upgrade it to OS X 10.5.8, although that can be pricey since Apple doesn't sell it anymore.