Since the AVI format is one of the easiest to edit, it's popular for people creating presentations, video tutorials and other instructional films.
It's not very often though that you get a screen capture tool which actually captures video in this format. AviTools consists of two programs - AviRecorder to capture screen activity in AVI videos and AviBuilder to generate still images from such files. You can record absolutely everything that happens on your desktop and reduce the file size somewhat by reducing the sampling frequency (although you lose a bit of video quality too). There are three different recording modes in fact - Whole Desktop which will record anything on your entire desktop; Window which lets you choose a window on your desktop or Rectangle which allows you to draw a rectangle on your desktop that will define the recording region. Simply move the mouse cursor to one corner of the desired rectangle, press the left mouse button and drag the mouse to the opposite corner of the rectangle. Release the mouse button there. AviBuilder works in a similarly simple to use way - simply select the images you want to use on the timeline, the resolution you want to save them at and the sampling rate.
If you're making a video tutorial or need your screenshots in AVI format, this is the quickest way to do it.