You can do basically everything you can imagine, if you put your mind to it. You spawn in a map, and from there you have access to a whole gigantic menu which contains props from many ct maps, hl2 maps/stages and of course tonnes of other specialmade items.
Theres a new(well, 1 year old) addon in it, called wiremod, where you wire things and make it more advanced...How should I explain that. For instance: normally you place a rocket somewhere, and bind it to a key, so it launches when you press that key. The special about wiremod, is that you can sort of program your own things... One example, place a button with a determined value when pressed...The button sends that value through wires, to another chip/thing/whatever and it triggers certain events. Some things I've made myself: 10 second countdown explosive, all the maths done by wire-chips and gates, an automated hoverbot that follows people around(can of course add lasers on pubs
), an elevator and much more...I have to say it's well worth the £10 for GMod 10.