Just because people use things for illegal purposes doesn't mean that the thing itself is illegal. For example, you can drown someone in water, but that doesn't mean that you or water is illegal. In fact water is quite nice and makes other things be able to quench their thirst and live and stuff.
So anyway, .torrent is the extension for bittorrent files. Bittorrent is not illegal, it's just another way of transferring files. Plenty of free big files out there, like Linux distros for example. Blizzard uses it for their WoW update thingy even.
Bittorrent can be nice for large files because everyone that downloads a file can also send it to others, so you can download parts from people close to you or even many places at once and get potentially greater speeds without having any negative effect on the bandwidth of whoever hosted the file originally. When you download you help everyone else download too, for everyone's benefit. (Bit of a simplification there, but essentially what it is.)
To download files with bittorrent you need a bittorrent client. The original one is at
bittorrent.com, but there's many others too with various features.
