Sharing is a social activity and hence its there on the internet also. Some forms of sharing files are considered illegal, but lets not discuss about that. Suppose you need a file, it may be a movie, mp3 or some software. Lets take for example, the latest Micheal Moore documentary. Its almost certain that many people already have it on their computers.
Torrents are a means for them to share the file so that others on the internet also can download it. Now whats so special about it ? We all can download files from a server, whats the difference when we are using torrents ?
When you are downloading the Micheal Moore documentary from a server, the entire 700MB is served from the same server. When 1000 people download the file, 700 X 1000 MB gets transferred from the server. 700 GB is a lot of data transfer. And many server admins might not like the idea since it will cost them a lot.
Torrents distribute the downloading process. If I am downloading a file using the torrent network, Im not downloading the file from a single server. Instead I am download various parts of the file from different users on the internet. My download client will rejoin the pieces when all pieces are downloaded.
Considering the previous example, if the same file is distributed using a torrent network the total usage on the server is drastically reduced. Every downloader acts as a server also distributing the pieces he downloads to other new downloaders. So when 1000 people have downloaded the file, we have 1000 servers for the same file.
Torrents reduce the load on the server. It improves the availability of the file since many people will be having the same file and is sharing it. So even if some of the servers go down, we will still be able to download the file from other servers/downloaders.