The image you see on a computer screen is a result of two factors, resolution and bit-depth.

Resolution refers to the dimensions of the displayed image expressed as the number of pixels wide and high (e.g. 640 x 480) and is independent of the monitor size (so that a 15" monitor could display an 800 x 600 resolution and a 17" monitor could be set to 640 x 480). Common resolutions are 640 x 480, 800 x 600, 1024 x 768, and 1280 x 1024, etc. For each step up in resolution a larger amount of pixels can be displayed, but any given image will appear smaller (imagine a 640 x 480 image taking up the whole screen at the lowest resolution, but only about a quarter of the screen at 1024 x 768).

Bit dept,( or color depth) is the number of colors that can be displayed per pixel and is the result of a mathematical formula: for example, If each pixel on the screen is represented by 8 bits of data, and a bit can have 2 values (on or off), then there are 2 to the 8th power, or 256 different distinct colors. Doubling the bit-depth to 16 bits results in 65, 536 colors that can be displayed. The chart below shows the common bit-depths available:

Video RAM (VRAM) is the memory of your computer dedicated to displaying images. The maximum resolution and bit-depth that your computer can support is a function of the amount of VRAM available, and are interrelated.

For example: a 640 x 480 resolution at 2 bytes per pixel("16 bit" or thousands of colors) would need 1 MEG of VRAM (640 x 480 x 2 bytes/pixel = 614400 bytes, divided by 1024 (1k = 1024 bytes) = 600 K). The same amount of memory could support a higher resolution (832 x 624) but at a lower bit-depth (8 bit or 1 byte/pixel.

The chart below shows the number of MEGs of VRAM necessary to support a given combination of resolution and bit-depth:

As can be seen, a computer with 2 MEGs of video memory can support any combination of bit depth and resolution indicated by the yellow boxes, and would have to choose between a larger screen resolution (1024 x 768), or bit-depth (millions of colors) at the highest settings. Also, monitors may have a maximum displayable resolution, regardless of the VRAM available.

The "base" configuration for a computer with a color monitor is 640 x 480 with 256 colors. The exact nature of the 256 colors differs between PCs and Macs, so that there are actually only 216 colors that will display exactly the same on both, known as the "web safe" palette. Images saved with millions of colors create larger file sizes than ones created with fewer colors, but when seen on a monitor that displays only 256 colors look the same AS IF it were saved with only 256 colors.

Most computers today support at least an 800 x 600 resolution and thousands of colors, so the need to limit images to 256 colors has decreased. However, with a new crop of PDA handhelds displaying black and white or 256 colors, anyone designing for this audience will also need to take note of the web safe color palette.