How do you compare pixels?

Hey guys...i am in urgent need of help..any help would be appreciated...
i am currently on a motion detection using a webcam project..now i am stuck at the point where i need to compare pixels from the webcam to detect motion... does anyone know how this can be done in vb.net?.

I usually bitblt the image to a memory bitmap. Then you can use the Bitmap Class's GetPixel() function.

could you explain bitblt a bit more?..or if you have any coding examples for this bitblt?... thanks