In short, XMPP is a protocol (like SIP). XMPP was majorly used for presence and Instant Messaging. But it is being expanded to other areas like call signaling control.
XMPP is getting more and more popular in Unified Communication products like CUPS (Cisco Unified Presence), Cisco Quad (SharePoint-like product), Contact Center, etc.
Since it's a direct competitor with SIP, frequently asked questions would be:
- Shall I learn XMPP?
- Will SIP be replaced by XMPP?
- XMPP was built from ground to support presence and IM
- XMPP is "eXtensible"
- XMPP is being (or has been) adopted by many big vendors like Cisco, Microsoft, Google, etc.
Anyway, you should take an hour or two to understand XMPP so you can understand what it can do (or do better) on different systems.