Unfortunately, this usually isn't possible. While most laptops have video outputs to send their video signal to another source, laptops generally don't have video inputs to take in a video signal from another source.
Something that is possible is to remotely control the desktop from the laptop, so whatever is on your desktop's screen goes on your laptop's screen. LogMeIn is a popular one. The downside to those is that things like videos or 3D graphics can be slow and choppy on the laptop in that case.
