Joltid PeerCache (JPC) is a device employed by ISPs to reduce the huge external network bandwidth required to support today's P2P networks. It basically acts as a caching web proxy (like Squid), only for P2P file data. Clients within the ISP's internal network use the cache to help coordinate downloads from external peers, allowing them to efficiently share downloaded data amongst each other, using the ISP's high-speed internal network, rather than relying on the significantly slower outside network. This saves ISPs from wasting a lot of expensive external bandwidth on duplicate P2P data, and allows their customer clients to download at blazing fast speeds using the much cheaper internal network bandwidth.
Azureus employs a built-in plugin, called JPC, to communicate with Joltid's ~PeerCache servers, allowing users behind a supporting ISP to run BitTorrent downloads via the cache.
The JPC plugin is safe in the way that your ISP won't know what you are downloading and can't use it to spy on you. The JPC plugin can be disabled through Azureus Tools > Options > Plugins > JPC.
For more info read http://www.joltid.com/index.php/peercache/
ISPs with a JPC
Does your ISP support JPC?
You can find out if you open the Plugins -> JPC view and have a look at the log file.
If you see something like following text your ISP probably doesn't support it. But you still can benefit from the JPC plugin when you're connected to a peer whose ISP has a peer cache.
Unknown Host : btcache.p2p Looking for : btcache.p2p.your.ISP.net Unknown Host : btcache.p2p.your.ISP.net Looking for : btcache.p2p.ISP.net Unknown Host : btcache.p2p.ISP.net Cache Not Found
Read the Azureus FAQ