The texture units make it possible to define the number of different textures that can possibly be accessed
(this is called a texture lookup or a texture fetching) at the same time in a pixel shader and thus from
a pixel pipeline. Currently, for nVidia and ATI, this number is 16. The texture units are managed by
the Texture Management Unit or TMU.
For ATI, since Radeon X1k (GPU R500+), the maximum size of a texture is of 4096x4096 pixels. In the preceding
series (GPU R300/R400), this size was of 2048x2048 pixels. For nVidia, all GPUs support textures of 4096x4096 pixels since
at least the NV20 (Geforce 3) and maybe even before.
On the other hand in terms of the vertex processor, the number of texture units is limited to 4 at the
maximum (for the nVidia cards).
See the following tutorial to obtain examples of pixel shaders making textures lookup:
The Art of Texturing Using The OpenGL Shading Language.
But to quickly summarize, textures lookup are done via an instruction of the texture2D() kind.