A hybrid disc usually refers to a DVD or CD-ROM that contains multiple file systems such as ISO 9660 and HFS (Mac OS).
It can also refer to multisession CD-R/W discs.
Lastly it may refer to an interactive CD-ROM that pulls dynamic content from the Internet, such as web pages.
Increasingly it may refer to SACDs which are 'Super Audio CDs', a higher quality audio disc format. --thx ReiTeoi!