The purpose is to create networks using GPG and RSA for secure chatting (which doesn't mean anonymously).
Anonymity can be achieved using proxies, Tor and secure networks.
Security → The protocol is based on public key exchange (currently only GPG) and signed data to verify the provenience of messages.
Decentralized → The network is based on nodes connected each other. Each node filters or drops data coming from unknown sources, i.e. from unknown/untrusted public keys.
Chat → The base of the protocol is for chat, but it's been designed to be extensible with other protocols.
Read more: sdchat