Factors to be considered when buying a laptop:

Laptops tend to lag behind desktops when it comes to upgradeability and features. So depending on what you need it for you should consider whether a desktop would be better equipped for your needs. Also you will not be able to upgrade your laptop when needed.

Portability: the whole point of a laptop is mobility so look at weight and size when purchasing.

Speed: laptops are generally slower then desktops. So again look at whether would be better suited for your needs.

Memory: it is very important you look at memory when first buying your laptop as most only have to expansion slots.

Batteries: Lithium batteries are superior to other kinds. The typical full size laptop will run between 1-2 hours on its battery. Some laptops can run between 6-8 hours on their batteries. If long battery life is import ant to you, enquire about battery life before you purchase your laptop.

Expense: in general laptops are more expensive then desktops. For the most basic laptops the cost starts around 300-400 euro.

Hard drive: if you need lots of space for storing files it is better to get a bigger hard drive but if it is not your only source you can get the minimum size.

Then depending on your usage look at features like CD-RD/DVD-RD, easy access ports like USB and screen resolution.