RAM shorts for Random-Access Memory, and it is the memory where software resides while it is running, along with the data it is using. Both OS and application software use RAM. RAM is a type of memory that is very fast, but is volatile, meaning all information is lost when electric power is removed. For this reason, it is useful only for temporary storage of data that requires fast access. A device will typically have RAM and some kind non-volatile memory to store a copy of all software and data that needs to be kept the device is powered off or that specific software is not running. Devices with more RAM can run more complex software and/or more applications at one time.
Most high end smartphones will use as big as 4GB of RAM. Till now the biggest RAM is up to 6GB.
Common RAM examples: 512M, 1GB, 2GB, 3GB, 4GB, 6GB.