It is sort of the opposite of vectorize. Vector images are made by a set of instructions ("Draw a line from this point to that one and then put a circle on the end"). Raster images are only made up of a whole bunch of little pixels that together look like something. You can have two images (one raster and one vector) that look identical, however if you zoom into the raster images you just see the pixels getting bigger. If you zoom into a vector images the lines will always look nice and smooth (assuming they were smooth to start with).
Here's my definition: "Rasterizing - what you have to do in order to be able to make that stupid shape do what you want." Seriously I don't seem to be able to change anything until I rasterize stuff...