What exactly is XBTC? What is xbtc? I saw many articles refered xbtc,just like "btc to xbtc". But there's no material telling what it exactly is.
XBTC is an asset representation of BTC in Counterparty.
Apparently, you could send Bitcoin to a Counterparty gateway, and get the same amount of XBTC back in turn. These could then be traded for other Counterparty assets. The operator of that gateway claimed that it was cheaper to trade thusly than on Bitcoin exchanges.

I've found an announcement for a XBTC gateway here: XBTC Gateway released. It appears to me that the service has since been shut down due to regulatory concerns.