A laptop is much harder to work with and upgrade. A desktop is very easy to work with and upgrade. To me, that is the basic difference. Also, laptops are more expensive, and some have overheating issues. There are pros and cons to both and you just have to weigh which one suits your needs more. If you have the money, as I did, you could get both a laptop and a desktop to supplement each other. I'm on the go a lot, so I got a laptop. However, I like working with desktops, and overclocking, so I have a desktop which I do that on too.
In general, desktops are more future proof than laptops, but are far less portable, and consume more power unless you get a desktop with laptop hardware. Most Macs are like this.