The nature of RAM, or Random Access Memory, allows data to be quickly staged for later use as needed. This is typically flash memory, or non-mechanical which allows for great speeds and high availability. Due to the nature of this data, things like a reboot or a new application being opened may overwrite this data as its not intended to be persistent. NVRAM, or non-volatile RAM is the opposite. On an Apple computer, NVRAM is used to store very simple things like your last volume setting, your keyboard backlight level, whether or not bluetooth was enabled at the last shutdown etc. Because this information is stored very quickly on an onboard chip, the Mac can access this data right away instead of waiting for the operating system to boot up.