Glide was 3dfx's proprietary API for their line of Voodoo cards. Given 3Dfx's near monopoly on the market during 1998, this worked quite well into 3dfx's attempt to gain more market share.
Since the Glide API was, essentially, embedded into the hardware, this allowed for ultra-fast gaming since a HUGE load was taken off of the processor. Textures rendered with Glide usually looked breathtaking - stepping out of the spaceship in the original Unreal, for example, was a breathtaking moment. Glide was also easier to program for, compared to the clunkiness of Direct3D at the time
Indeed, many games were ported to Glide; Descent and Descent 2 both got a facelift, breathing new life into these games. Glide was THE standard to program for back in the day, ensuring 3dfx's eventual downfall when their OpenGL and Direct3D performance weren't quite up to par
Of course, there were disadvantages to programming in Glide - textures were rendered in 16-bit color, reflecting 3dfx's stance that 32-bit color was an overhyped gimmick. Driver updates caused texture misalignment in some games. And, of course, you were limiting yourself to a particular brand of card.
Nowadays, Glide is dead in the water. As Tim Sweeney said:
Glide is dead. Nobody is writing any new code aimed at Glide. There are some games on the market still taking advantage of it, so it will be a little while before the thing is fully buried. But I can assure you developers are doing their best to shovel dirt on the grave, and if we ever see the deceased try to claw its way out, we will whack it back down.