Dolby Labs is one of the most significant companies in AV history. Some argue that the development of Dolby Pro Logic and its application on VHS tape is the true spark that started the home theater business.

Today Dolby Labs designs and implements some of the best lossless surround sound found on formats like Blu-ray with Dolby TrueHD.

Dolby also licenses many technologies (like Meridian's Lossless Packing or MLP) to other companies for use in AV and other applications.

HomeTheaterReview.com has explanations of the following Dolby technologies:

Dolby Pro Logic
Dolby Digital (AC3)
Dolby TrueHD

Dolby's main competitor is DTS.