Despite the fact that a fair usage limit on something labelled 'unlimited' is entirely antithetical, there are performance reasons behind providers' policies which may impact your internet usage.
Customers share internet connections. The number of people sharing a connection is called the 'contention ratio', and it can be up to 50 people per line. So if one user is constantly downloading, it will slow the connection for the other users who are on at the same time.
Fair usage policies limit the usage of one user over another at peak times so that the performance is not affected for the rest of the group. Providers tend to group people according to usage, so people who download all the time are grouped together.
These restrictions are imposed to ensure that the best quality of broadband is available to the maximum number of customers. So even though you have a 24Mbps connection with unlimited downloads, your speed may be heavily limited at peak times.
This is frustrating for a lot of people who have specifically bought high-speed packages because of their usage patterns. This kind of advertising is aimed at a heavy user and encourages people to get super speeds and unlimited allowances.
Check with your ISP to see what your contention ratio is and what they classify as fair usage.
Heavy or excessive users are generally those who use peer-to-peer (P2P) or file-sharing software. They are usually people who upload and download large files, such as movies and music, on a daily basis. These users take up excessive bandwidth, slowing the connection down for everyone else.
Generally, after a couple of months of really heavy use, ISPs will notify you if they feel that your usage is excessive. This notification will usually point you in the direction of their specific policy on 'fair usage' and suggest that you refrain from uploading or downloading large files at peak times.
Day-to-day surfing, checking e-mails and occasional downloading will not get you into trouble with your ISP. Downloading a 3GB movie every day is almost certain to, however.
The key is to keep an eye on the amount you're downloading. Then, if you're classed as a heavy or excessive user, restructure your internet usage pattern so that you are not uploading or downloading at peak times.
If you don't take heed of their warnings, your ISP will slow down the speed of your connection at peak times. In serious cases, your ISP may terminate your internet connection.
Be warned - unlimited is not quite unlimited.