You probably have a video camera that you use from time to time, and eventually you’ll need to upload them to your computer. If you have a Mac, this does not always go smoothly because of file codecs. If you open up an AVI with Quicktime, it will likely play just the audio and leave the video box black.
Here’s a simple solution: Convert the files to mp4 with Handbrake. Handbrake is freeware that can be downloaded @ handbrake.fr. It is commonly used to rip DVDs to mp4 files, but it can also be used as a video converter.
First, click on the source button, then select your AVI. Then select the destination in the file bar. Select the codec (you probably want mp4) under “Output Settings”. Click the “Start” button, or “Add to queue” if you are converting many files.
Using HandBrake is best for converting videos to give to friends or put on DVDs. It may take too long to convert all of your video files, so you could just play them with VLC media player (free, videolan.org)