The Graphics Processing Unit in the Sony PlayStation is a custom 2D triangle renderer.
The GPU provides 1MB of VRAM, which is configured as a 1024x512 16-bit framebuffer, although the framebuffer can be used for 24-bit output as well. The VRAM is shared by the framebuffer (which is usually double-buffered to prevent tearing), texture assets and color lookup tables. There is no Z-buffer, but there is a 1-bit mask bit for drawing.
PSX games generally use the CPU's GTE (Geometry Transformation Engine) to compute and project vertices and a special DMA mode to create a linked-list ordering table to render polygons. Rather infamously, the PSX GPU is not capable of perspective-correct rendering, causing textures in scenes to "wobble".