RAM: (random access memory) is a type of computer memory that can be accessed randomly; that is, any byte of memory can be accessed without touching the preceding bytes. In other words, RAM is the place in a computing device where the operating system (OS), application programs and data in current use are kept so they can be quickly reached by the device's processor. RAM is much faster to read and write than other kinds of storage media in a computer. Data remains in RAM as long as the voltage supplies it maintains on. When the computer is turned off (power off), RAM loses its data. When the computer is turned on again, the OS and other files are once again loaded into RAM, usually from an HDD or SSD.
There are also different kind of RAMs so called Dynamic Random access memory DRAM and single data rate SDRAM.
RAM usually differes due to their transfer rate of bits.
With data being transferred 64 bits at a time, DDR SDRAM gives a transfer rate of (memory bus clock rate) × 2 (for dual rate) × 64 (number of bits transferred) / 8 (number of bits/byte). Thus, with a bus frequency of 100 MHz, DDR SDRAM gives a maximum transfer rate of 1600 MB/s.