First off, I'd recommend hitting F1 and reading up on it in Maya's help files (rendering section) as it gives you enough info to get up and running.

The gist of it is that you create the two shaders that you want to layer on top of one another, then connect each one to a blendcolor node (mmb drag each shader onto the blend node's color1 and color 2 attributes -- whatever's plugged into color1 will be the topmost material). Then you connect the blend node to the color of another material's (blinn, lambert, etc) color attribute. If you look at the blend node's attributes you can adjust the blend between the two materials using the blender slider or use an alpha map to drive it like in the example I've attached. It's good for making ruinous walls, moss, and other things like that. The example is very simple but you can get as complex as you want (adding the final shader to a layered shader, etc. etc.) It's kinda like building with tinker toys...you just keep adding and building little pieces until the whole room is covered in nodes and connections. On an ending note, in the example, the alpha map is also driving the bump of the final blinn.