Standard GIF - type 87a or 89a non-interlaced - can contain as many as 256 different colors and loads from top to bottom in a linear fashion. This renders the picture as it will ultimatly look line by line.

Interlaced GIF - type 87a or 89a interlaced - same as above except this image will appear to melt onto the screen slowly until the whole picture is clear. This may be preferable for users while waiting for a page to load.

This what a GIF could look like before it is made transparent. It's a good idea to use a color for the background which is a stark contrast to the colors in the object(s) you want to be shown. Here the background was set to a shade of purple since there were no colors close to it in the image of the mailbox or the text.

The same GIF as above but made transparent using Adobe Photoshop's GIF export filter. What you get when you make that color purple transparent. These kinds of GIF's can really enhance a web page's design.

Animated GIF's take longer to load because they are a series of GIF images loading one after the other. Once the whole file has been read though, the GIF usually picks up speed and runs smoother. This revolving globe is an example of a non-transparent multi block or Animated GIF.

You can also make an animated GIF transparent as seen in this fire hydrant and dog animation.