VGA terminology

Resolution

The resolution of a screen is the amount of distinct pixels that can be shown horizontally and vertically.

A resolution of 160x144 means that the display is 160 pixels wide and 144 pixels high (which results in a total of 23040 pixels).

Pixel

A pixel is a unit which can output light of a chosen wavelength with a chosen amplitude(intensity).

In case of colour displays, the pixel is a collection of 3 subpixels each responsible for either Red, Green or Blue (RGB). (alternatives exist but won't be discussed)

Refresh Rate

The refresh rate of a display tells us how often the entire display gets updated per second (Hertz).

It is necessary to update the display periodically as displays expect this. (60Hz - 100Hz is typical nowadays)

A clocked signal is responsible for letting the display know it's time to refresh, this signal is referred to as vertical sync or VSYNC.

Horizontal Sync

Horizontal Sync or HSYNC is a clocked signal which tells the display that we want to start sending pixel data at the start of the next line.

To show what I mean, here is an example: (Resolution: 3x3)

[1][2][3] HSYNC!

[4][5][6] HSYNC!

[7][8][9] VSYNC & HSYNC! (simultaneously)

The numbers display the order in which the pixels get updated.

Bits per pixel

The bits per pixel or BPP tells us how many different (colour) shades a single pixel can display.

The gameboy for example has 2 BPP which tells us that every pixel can have 4 different shades. (00, 01, 10, 11 -> white, light grey, dark grey, black)

To calculate the amount of shades (/colours), we simply calculate 2^BPP.

Pixel clock

This clock is responsible for sending the pixels to the display in a timely fashion so that the display will position the pixels correctly.