The hard drive stores data to be retrieved later
The Mother Board hold most the other pats of the computer and provides along with other sub systems in it.
RAM is random access memory, used for any function that need memory to do.
CPU does the processes what the computer will do.
The heat sink take in heat and disperses it so the computer does not overheat
The power supply take in power and converts it to a lower voltage that the computer can use safely
The video card take the computer code and make it into an image that can be displayed on a monitor.
The video card is not necessary if you have integrated graphics.
The clock battery keeps power to functions that need to be on when the computer is off, like the internal clock.
Without the battery, your computer could have problems.