Peercache is a commercial p2p caching system made by a company called Joltid. The server software is closed source and costs money. The main purpose of peercache is to reduce the amount of traffic an ISP has to exchange with other ISP's.
eMule has supported peercache since version 0.43
By default every two days emule attempts to resolve the domain name edcache.p2p and other names based on the name of the isp such as edcache.p2p.adsl.isp.com and edcache.p2p.isp.com
This can be turned off in extended settings.
There are reports that people who's ISP's use peercache have not be able to download files larger than 4GB unless peercache is turned off.