How much does an average computer tower weigh?

The main components are:

Motherboard - the 'brains' of the computer - holding the CPU, memory and various sockets and chips.

RAM - the temporary 'memory' - which is wiped when the power is turned off

Hard-drive - a mass-storage device containing all the computer programs the user wants to use. It's contents are not wiped when power is turned off.

Optical drive - an input/output device that can be used to read data from the computer to be used on another machine - or be used to load programs onto the computer.

Video card - to provide an image for the operator to see what the computer is doing.

Power supply - which steps down the mains to a more usable voltage for the computer to use.

There are other minor components - but those are the main ones.