Motherboards come with four memory slots to install RAM. These slots are typically used for DDR or DDR2 memory, but they can also hold other types of memory, like PC-3000 SDRAM.
The main reason motherboard manufacturers include four memory slots is to support multiple versions of DDR or DDR2 RAM. Older motherboards may only support a single type of memory, so if you want to upgrade your RAM, you need to buy both new types of memory and replace the old ones on your motherboard.
Newer motherboards usually come with four slots for DDR or DDR2 RAM, but they may also have more slots for additional types of memory. This lets you upgrade your RAM without having to buy a new motherboard.