DCC – Direct Client to Client (previously Direct Client Connection). DCC allows users to chat and send and receive files privately. A connection is made between the two client computers, completely bypassing the IRC server. When initiating the connection, the originating client sends the connection request directly to the target, bypassing the IRC server.
More info on Direct Client-to-Client: https://en.wikipedia.org/wiki/Direct_Client-to-Client
CTCP – Client-to-Client Protocol. Another way to initiate a DCC connection is using CTCP. When using CTCP, the originating client uses the IRC network to send a connection request to the target client. Once the connection is made, the IRC server is no longer involved.
More info on the Client-to-Client Protocol: https://en.wikipedia.org/wiki/Client-to-client_protocol