3D computer graphics are graphics that use a three-dimensional representation of geometric data stored in the computer for the purposes of performing calculations and rendering 2D images.
3D could mean 3D graphics. 3D graphics usually involve a lot of matrix manipulation and vector mathematics. These matrices will be both affine transformations for manipulating the objects in 3D space (translation, rotation and scaling) and perspective transformations for generating the 2D image required for display. More information about 3D graphics at Wikipedia page of 3D graphics
3D could also mean Stereoscopy. Stereoscopy (also called stereoscopics or 3D imaging) is a technique for creating or enhancing the illusion of depth in an image by means of stereopsis for binocular vision. To generate stereo images two normal displays are generated offset from each other by the distance between the eyes. These are then combined on the display in one a variety of ways (LCD shutter glasses, polarised projections etc.) to generate the 3D effect.
More information on Stereoscopy at Wikipedia page of Stereoscopy
