Extensible Messaging and Presence Protocol (XMPP) is a communications protocol for message-oriented middleware based on XML. Unlike most instant messaging protocols (ICQ, Skype, Telegram, WhatsApp, etc), XMPP is defined in an open standard and uses an open systems approach of development and application, by which anyone may implement an XMPP service and interoperate with other organizations' implementations. Learn more about Jabber/XMPP.
Is it safe?
Yes, Jabber is the most secure from popular instant messengers. First of all XMPP is distributed protocol, messages don't saved at the one server/company (like Telegram, ICQ, Skype and others). XMPP-server has data about its JIDs only and doesn't know anything about JIDs of others XMPP-servers. Jabber supports the most modern and strong encryption algorithms, like OTR and GPG.
Apps for XMPP
The most popular free applications (Jabber-clients) for XMPP are: Psi, Psi+, Pidgin, Miranda, Gajim, SJ. Most of these clients are cross-platform. This means that clients are available for Windows, Linux, Mac OS X, Android and iOS.
How to join?
You can register via web (form above) or via Jabber-client (in-band registration).