shaders are sorta kinda lightmaps on steroids, but more... but unlike lightmaps they are calculated on the fly
they were a big deal new gee-whiz stuff when Quake3 came out, here check this:
http://qeradiant.com/manual/Q3AShader_Manual/
also some info on how they are handled hardware-wise
http://www6.tomshardware.com/graphic/20020116/
But, the use of shaders in Generals is kind of curious to me, considering the very limited lighting effects and the low-detail infantry... I'd like to know exactly what they are using it to do. Maybe mainly for the terrain?