You have normal RAM, DRAM, SDRAM, DDRAM, and many more.

To keep this all easy, most motherboards support 256 MB RAM chipsets, like mine. My computer has 320 MB RAM with 256 MB and 64 MB chipsets. There's another slot for another 256 MB.

Anyway, if your computer is a Compaq Presario, you have most likely SDRAM, which can be easily bought, sine it's the most common RAM available. You should be able to find the details here: http://www.orderingmemory.com/compaqconsumer/
It also has RAM details for other computers, such as Apple and Intel.