Component of Computer System
3. Memory
There are two types of memory in the PC:
1. RAM(Random Access Memory)
2. ROM(Read Only Memory)
RAM
It is used to quickly access the data. We can read and modify the data in RAM. It is a volatile memory it losses the data when the power is switched off.
RAM is very essential part of the computer also called main memory. All the programs and applications are run in the RAM . Larger the RAM more programs PC can support and speed up the pc.
RAM can be classified in two types
1. Static Ram(also called cache memory)
2.Dynamic RAM
The most commonly used RAM is Dynamic RAM. There are two types of DRAM:
1. SIM(Single in-Line Memory Module) it has 30 or 70 pins. Sim rams are outdated ram. They are only used in olden computers. The capacity range of SIM ram is 1-16 Mb.
SIMM( 72 pins)
2. DIMM(Dual in-Line Memory Module) it has 168 pins. There are two main types of ram in DIMM one is SD and second is DDR
SD ram have two cuts and it is generally used in old motherboards. Some PIV motherboard also uses SD ram. SD rams are also costlier than DDR ram 
SD RAM (168pin)
DDR ram have one cut and is used in current boards. Now a DDR rams have became very cheap.
DDR RAM
Through Input Output (I/O) ports we can connect device to the computer.
There are two types of ports external and internal.
Internal ports are used to connect internal devices such as hard disk, CD-Rom etc.
These devices are connected through port connector cables.
External ports are used to connect external devices example keyboard, mouse, printer, joystick, external modems, monitor, speakers and all the external peripherials.