Message Queuing Telemetry Transport, or MQTT, is a lightweight signalling and message transport protocol originally developed by IBM for their WebSphere suite. and extended to work with sensors. A history of its lineage and examples of use in today's web ecosystem can be found at Wikipedia, or the MQTT website.
One of IBM's goals was to make a protocol that would allow for easy web interfaces with sensors and other "real-world" devices, much in the way that simple REST APIs made web mashups possible (and massively popular) in the early 2000s. In fact, Facebook messenger uses MQTT as the underlying protocol, and has been recently championing it as the protocol that will run the future Internet of Things.
From a technical perspective, MQTT is well designed and documented, and is particularly well suited to low bandwidth applications like those where connectivity is provided by a cellular or satellite channel where bandwidth (and possibly total transfer allotment) is at a premium.
In WireSpring's testing of signalling protocols for various M2M projects (some involving multimedia, others involving only low bandwidth messages), MQTT came to
be about 1/3 smaller than its main rival, XMPP when using equivalent
binary compression (since it was available server- and client-side on
our projects). However, XMPP has MANY more bells and whistles built-in
(for example, well defined message formats). Consequently, if the development team isn't careful, and requires even a mosdest amount of functionality not explicitly designed in to the MQTT protocol, it is possible -- or even likely -- that the resulting MQTT would be larger than a comparable XMPP solution.
Cisco has a Cisco-slanted but not-too-terrible baseline comparison of pros and cons here:
Beyond MQTT: A Cisco View on IoT Protocols.