Ram itself has three basic types:

Sodimms for laptops

Rimm for Servers

Dimms for Desktop/Workstations

Sodimms- have a few different flavors and the most prevelant at present are the DDR and DDR2's, with DDR3 hot on their heels.  When purchasing memory for your laptop make sure you are buying the correct type for your computer as none of these will fit in the incorrect slots.  If you purchase the wrong sodimm you will have ram that will not work and most times it is non-refundable.

Another things to watch for is the density of your computers ram.  Some laptop can only use Low Density ram, and some can only use a maximum of 64x8 high density ram.  While new to the market is the 128x8 and 256x4 desnities.  If you purchase these types odds are it will fit in your slot however your computer will just report it as bad although it is good memory.  The reason for this is that the chips are 128mb chips and the computer can only read a maximum of 64mb per chip.  Again, it will appear as though the memory is dead, when it is good but will not work on a system that have low density requirements.  If you call and verify your computer with the company do not ask if it can run high desity ram as they may tell you yes, but you did not ask the key words.  Can my computer run 128x8 high density ram as 64x8 is also considered high density ram.  Many manufacters will tell you that the computer can use 128x8 and larger ram however, you are talking to a call center and most of the people working in a call center do not even know what density means. They really are not techs, but just handed a script and told how to answer questions.  If they do not know the answer they will send you in a loop and hand you off to another call center individual.  How do we know this?  We worked in these call centers and watched it happen.  So be leary.