An archive is a collection of information and media much like a library or interactive storage facility. It is a necessity for an on demand media service to maintain archives so that users can watch programmes that have already been aired on standard broadcast television. However these archives can vary from a few weeks to months to years, depending on the curator and what programme it is.
For example, BBC iPlayer offers most of its programmes for 30 days after their original air date on the BBC. However some special programmes such as Panorama are available for an extended period because it is a factual programme and is highly watched and so is worth the extra money needed to host it for longer.
In contrast 4OD channel 4's on demand service offers many of its much older programmes as well that were originally aired years ago. An example of this is the comedy "The IT Crowd" where users can view the full series on the internet player. The same is true for other hit channel 4 comedies such as "The Inbetweeners" and "Black Books".
Having an extensive archive however can bring problems along with benefits. Large archives are expensive to maintain, server farms and mass storage is needed along with ample bandwidth to transmit it all. Vast archives can be hard to catalogue and sort so that it is accessable to users.
The benefits in most cases outweigh these problems. This is because large archives bring in far more users who in turn watch more media, leading to a wider audience base and more advertising revenue. Large archives will also mean the user will spend more time on that website rather than a competitors, leading to starvation of demand for the competitors.