One of the unsung benefits of a modern motherboard is its ability to monitor and maintain a steady voltage to each component. The power supply will supply power within certain tolerances, and the motherboard refines these in order for each component attached to it to get exactly what it needs. Each element is extremely sensitive to voltage fluctuations and the motherboard does a lot to prevent voltage spikes which could damage the components.
The board will also contain a BIOS, the Basic Input/Output System. This is held on a chip on the board and contains the firmware necessary for basic motherboard functions to take place. When you first boot a computer, the black screen with the processor and memory details on it is the BIOS loading. The job of the BIOS is to power, test and then initialize system components ready for the operating system to pick them up.
The Process, or How it Works
Every time you press a key, the keyboard sends the command to the operating system. For example, if you press a letter key in Notepad the operating system sends a command to the processor. If it isn't busy, the processor decides what to do with the command, then routes that instruction to the correct component, in this case the graphics card. If it's a simple key press the instruction will be to display certain pixels on the screen in a certain position to resemble the letter.
That is the most basic process, but the method remains the same whether you're pressing a key, playing a game or computing Pi to the nth degree.
Delays can occur if the processor is busy doing something else, or the component is already engaged in another activity. Pauses after issuing a computer command can often be blamed on this. A CPU uses "cycles", each completed instruction is one cycle, the speed of the processor directly influences the amount of cycles is can process per second. Things get complicated further when you talk about multi-processor CPU's.