RAM (Random-Access Memory) - definition
This is the memory where the software resides while it is running along with the data it is using. RAM is used by both OS and application software.
RAM is very fast but volatile, meaning that all information is lost when electric power is cut off. That makes it useful for temporary storage of data that requires fast access.
Normally, devices with RAM also have another type of storage memory (flash memory or a hard drive) that stores the information while the power is off.
Devices with more RAM can run more complex software and multiple applications at the same time.