5.1 describes a theatrical setup of five discrete channels of audio and a
Low Frequency Effects/Energy (aka LFE) channel to handle deep bass. I say discrete, due to the fact that each channel can play a different sound independently of one another. The .1 aspect of the setup describes the LFE, since it requires around a tenth of the bandwidth of a regular channel of audio.
A typical setup consists of a Left, Right, Center, Surround Left and Surround Right set of full-range speakers and a dedicated subwoofer to handle the LFE.
Media that have the capability of 5.1 channels of discrete sound include Dolby Digital, DTS Digital Surround, and 6-track 70mm prints. I am unaware of SDDS' ability to replicate the 5.1 setup, since it uses 7.1 natively. However, it's natural that it could just play silence in the additional 2 channels that it provides.
THX EX, otherwise known as Dolby Digital 6.1 EX extends the idea of 5.1 channels by adding an additional channel behind the viewers to complete the enveloping 360 degree surround sound experience.