There are two types of storage, primary and secondary storage. Primary storage deals with temporary storage and is usually called "memory" or "main memory". Secondary storage deals with permanent storage and is usually called "storage". Most memory is referred to storing media, documents...etc on chips, whereas storage stores it on disks or tapes. Memory (primary storage) is stored on RAM chips or Random Access Memory chips. Whenever you open a file from the secondary storage it is opened on the RAM and allows you to make amendments to it. When you save the file it is then moved back to the secondary storage and saved on the hard drive. Basically you can think of the secondary storage as the refrigerator and the primary storage or RAM as the counter or stove in which you cook. There are different types of RAM some which include DRAM, SDRAM, SRAM and DDR - SDRAM. DRAM is abbreviated for "dynamic ram" and is constantly refreshed by the PC to keep its contents. SDRAM is abbreviated for "synchronous dynamic ram" and is synchronized by the system clock. This allows it to be faster than DRAM and the speed is measured in megahertz. SRAM is abbreviated for "static ram" is faster than DRAM and keeps its contents without having to be refreshed by the PC. DDR - SDRAM is abbreviated for "double-data rate synchronous dynamic ram". It is the most current RAM, it is used commonly in PC and Apple's and is also measured in megahertz.