General Information

XMPP is a communications protocol which enables the near-real-time chats between any two or more network entities. That means it’s like a skype or facebook chat between two or more users. It was originally named Jabber, a name which sometimes still gets used for it.

Following this guide you will be able to send end-to-end encrypted messages in real time for free.

OTR(Off the record)
Pidgin (formerly named Gaim) is a free and open-source multi-platform instant messaging client. It has support for many instant messaging protocols, allowing the user to simultaneously log into various services from one application. That means you could chat with your facebook / google talk / AIM friends using only Pidgin and not visiting the website itself (e.g. facebook.com).

Pidgin is widely used for its Off-the-Record Messaging (OTR) plugin, which offers end-to-end encryption. For this reason both (Pidgin and the OTR plugin) are included Tails and you just have to set it up correctly. However your chat partners have to have the OTR plugin too (Pidgin is not necessary, they could use a similar tool) in order to be able to chat with you this way.

The OTR plugin ensures the messages cannot be recovered by a third party because it uses Perfect Forward Secrecy. However as always your other chat partner could always keep logs of your conversation without you knowing or be compromised.