Adding more memory is the easiest and normally cheapest upgrade you can make for your Laptop or Notebook. Upgrading options are far more limiting than compared to a standard desktop machine, mainly because it is so hard to get inside and remove parts without taking out the motherboard and all of the connected casing.
Usually there is room for an additional two modules to be installed or course if one is already used you could replace this with a larger version to improve the amount of memory.
Laptop Memory is smaller than the normal type used in PCs and Macs. Just like the bigger desktop memory Laptop memory comes in the same standard DDR, DDR2 or DDR3 for newer systems and SDRAM for older they are usually called SODIMM which stands for small outline dual in-line memory module.
The 200-pin SO-DIMM if the notch is located further outboard, it indicates the DDR class of memory. When the notch is located towards the centre of the board, it indicates DDR2. These two types of memory are not interchangeable and thus have slightly different notch locations that prevent cross installation.
Again just like the full size modules you can install faster speed DDR/DDR2 memory into your system if your motherboard supports the type and likewise it will run at the lower speed if used with a slower module.