Memory comes in a variety of formats and uses.
RAM stands for Random Access Memory, and is a type of memory that can be both written to and read from. Most RAM types are Volatile, so the contents of RAM are lost when the system powers down. One exception is NVRAM, otherwise known as Non-Volatile Random Access Memory.
The RAM is the temporary holding place for data. When available RAM space runs out, the OS pages out data to the Hard Drive. This results in a loss of performance, so, the more RAM you have in your system, the less often your system has to use the hard drive as a temporary holding area.
RAM comes in a variety of types. First, you have Dynamic RAM, abbreviated DRAM, and then you have Static RAM, also known as SRAM. DRAM's contents refresh frequently, while the contents of SRAM do not need to be refreshed. SRAM's read access times of around 10 nanoseconds are faster than DRAM's average of 60 nanoseconds. SRAM is also much more expensive to produce, and it usually limited to small portions of memory, such as Cache memory.