Transform & Lighting Engine
'Transforming' is the very FP-calculation intensive task to 'transform' the 3D-scene with all its objects, called 'world-space' into the 'screen-space' that we are looking at. 'Lighting' is pretty self-explanatory, it represents on optional stage in the 3D-pipeline that calculates the lighting of objects in relation to one or several light sources. Lighting is just as transforming a pretty FP-calculation intensive task. Both tasks used to be executed by the CPU, putting rather heavy strain on it. The effect was that the 3D-chip was often in the situation that it had to wait for the CPU to deliver data (e.g. CPU-limited 3D-benchmarks) and that game developers had to restrict themselves to less detailed 3D-worlds, because heavy polygon usage would have stalled the CPU.
GeForce and GeForce2 GTS put an end to this dilemma, their T&L-engine can take the strain off the CPU, can keep the 3D-pipeline from stalling and allows game developers to use much more polygons, which automatically results in greatly increased detail. At the same time the CPU can dedicate itself to a more complex game AI or more realistic game physics. GeForce could transform, clip and light 15 million triangles/s, and GeForce 2 GTS pushes that number up to 25 million triangles/s.
AGP4x Fast Write
The 'Fast Write'-feature enables the CPU to directly write to the graphics card's frame buffer without taking a detour through system memory, and it's supposed to be up to 30% faster than 'normal' AGP4x. So far there's not too much to see of that performance advantage, neither on Intel based platforms, nor on VIA based ones.
Some More ...
GeForce2 also supports 'vertex blending', which basically joins different pieces of geometry with each other, as e.g. required in joints of characters or animals. 'Particle Systems' is another feature included into GeForce2, and NVIDIA has a rather impressive demo for it. Particle systems could be very useful to display nice explosions (what's nice about an explosion?) or the fountain in Billy Christal's garden from 'Analyze This'. 'Per-Pixel Mip Mapping' offers you a more realistic 3D-world by applying the right mip map level of a texture at pixel rather than at polygon level. This way textured structures that run from close to distant can look more realistic, because it avoids the 'mipmap jump' between two polygons.