DC/DirectConnect - the name of the p2p system Jucy is made for.
Hub - Hubs are servers where your client connects to. They forward your searches to other clients.Also they help establishing connections to other clients so your downloads can start. And provide facility to chat in public (referred to as mainchat or just main) or in private (PM).
OP/Operator - Hubs are guarded by OPs. These are other users with special privileges that may kick or ban users they seem unfit for their hub. You can distinguish them from normal user as they have a yellow key beside their user icon.
Owner - The owner of a hub is usually the one hosting it. So he is the one that decides who is an OP in his hub.
Mainchat - or main the place where all the chatting takes place. Also a good place to ask any open questions you might have about DC.