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[Ofnuts]

There are three images modes (see the Image>Mode menu):

• RGB (aka "full RGB"). A pixel is defined by three color components (R, G, and B). This is the most frequent case, and corresponds to JPG and (most) PNG images.
  
• Grayscale: A pixels is defined by a single component (often called "Value"). Not to be confused with an RGB images where R=G=B. Although supported by the JPEG format, it is rarely used to publish images.
  
• Indexed: a pixel is defined by an index in a colormap. The colormap holds 256 colors maximum. This corresponds to GIF and (some) PNG images.
  

In addition, images can have an "alpha-channel" that defines how opaque the pixel is. Some image formats cna store it (PNG) and some don't (JPG). In indexed image, there is  no progressive opacity, it's fully opaque or fully transparent.