You can copy your entire iTunes library from the desktop to the laptop.
Or if the laptop already has an iTunes library with media on it, you can enable Home Sharing on both iTunes libraries.
That allows you to see which files the other library has that the one you are on doesn't, and allows you to copy those files from the other library to the one you are on.
So for example I have a desktop Mac and a laptop Mac.
Both have separate iTunes libraries, and both have Home Sharing enabled.
I can add media to iTunes on my desktop Mac.
Then on the laptop I can go into iTunes, then click on the drop down which gives you access to Music, Movies, TV Shows etc... at the bottom is my desktops library.
From there I can then see either all the items in my desktops library or just ones ones the laptops library doesn't have.
Then I select the files I want and click import. iTunes then copies across the files from my desktops iTunes library into my laptops iTunes library.
EDIT: This relies on both computers wing turned on with iTunes running and both computers need to be connected to the same network.