RAM
Random access memory contains the software and data structures that allow the router to function. The principle software running in RAM is the Cisco IOS image and the running configuration.

ROM
Read-only memory contains microcode for basic functions to start and maintain the router.

Flash Memory
Flash is primarily used to contain the IOS software image. Some routers run the IOS image directly from Flash and do not need to transfer it to RAM.

NVRAM
Non-volatile random access memory is mainly used to store the configuration. NVRAM uses a battery to maintain the data when power is removed from the router.

Configuration Register
The configuration register is used to control how the router boots up.

Interfaces
The physical connections to the external world for the router. These interfaces include the following types: Ethernet and Fast Ethernet, Asynchronous and synchronous serial,Token Ring, FDDI , ATM, Console and auxiliary ports.