System memory is where a computer holds the programs and data that are currently in use. The definition of the word "memory" has become somewhat blurred over the years because there are different types of memory that a computer uses. In general, "memory" refers to the system memory, or RAM, which holds instructions that the processor uses, and the data the instructions work with.
System memory is directly related to the performance of your computer. In some ways, it's even more important than the processor, because without enough memory, your processor cannot work to its full potential. Newer programs require more memory than older ones, so when buying a new computer, get the most memory that you can afford.
There are many different types of system memory available. Researching them will help you make an informed choice when you purchase your computer. Getting the right kind of memory for your system will also affect whether you can upgrade it later on, and by how much. With the right memory, you can move it from one system to another, or continue to use the same memory even after you upgrade your motherboard.
Types of memory include 168-pin SDRAM, 184-pin DDR SDRAM, 240-pin DDR2 SDRAM, and 240-pin DDR3 SDRAM. Capacity typically ranges from 128MB to 2GB with speeds covering PC 100, PC 133, PC 2100, PC 2700, PC 3200, PC 4000, PC2 3200, PC2 4200, PC2 5300, PC2 5400, PC2 6400, PC2 8000, PC2 8500, PC2 8800, PC2 8888, PC2 9136, PC2 9200, PC2 9600, PC2 10000, PC2 10400, PC3 8500, PC3 10600, PC3 10666, PC3 10700, PC3 12800, PC3 13000, PC3 14400, PC3 15000, and so on.