Dolby Digital and DTS - the two most common forms of multi-channel digital audio. Most movies today are encoded in digital surround (e.g., 5.1, 7.1, 9.1, etc.) and the way these are stored on DVDs or movie files is using either Dolby Digital or DTS technologies.
Yes, all Macs sold today have optical audio input and output. The mini-jack is dual-purpose. If you plug in a 1/8" connector, it sends out analog audio. If you plug in a Toslink cable with a miniplug, it sends out optical audio. If you have a receiver or amp that can take optical, then you can get digital audio from your Mac. Analog can only carry Dolby Pro Logic II (5-channel) at this time. Have a look
here for the details.