A torrent is a file hosted on the internet which contains information about other files which you are looking to download. The majority of torrents that you will find online are for the purpose of downloading movies, albums or games. To use a torrent, you will need a torrent client, the most popular of which is BitTorrent. There are also various sites which host torrents but finding them is at your own discretion as sharing them violates the terms and conditions of the forum.
The information from within a torrent includes where to download the file from, the files that are to be downloaded, and the file size. When you "open" this torrent in a torrent client, this information is read, the files are located and then downloaded to your computer. When downloading torrents, the information is collected from people who have the file on their computer. These people are called "seeds". You "take" a copy of the information from them. People who are also trying to get the file are known as "leechers" due to their nature of feeding from seeds. If you are downloading a file, you are a leecher until you have completed the download, at which point you become a seed for other people to leech from.
Torrents work on a torrent principle. This means that the more seeds there are, the quicker your download will be. This is common sense. If there are more people online to take the file from, you can acquire it quicker than if only a handful of people had it. In the same way, if there are a lot of leechers, the file transfer is slower. The download rate also depends on your internet connection and the size of the file. If the file is big, it will take longer to complete. If you have a fast internet connection, chances are, it won't take as long.
With regards to your original question, you can download The Sims 2. You would have to download the base game if you don't already have it; downloading an expansion pack will not work without the base game. As far as I know, it's about 2.2 GB big and on a slow connection, this could take several days to complete. On a regular broadband connection, you're looking at about 20 hours or more. I wouldn't recommend downloading it as you'd then have to download a key generator to validate your copy, and the risk of downloading a virus or trojan is high. There are also many things that can go wrong with pirate copies of gamesand there is no support available for you if something goes wrong with the game or your computer.
You can find more information on torrents
here and
here.