Smooth Playback is a webcomic about web video.
You've got a stubborn video file giving you trouble. What's the first thing you look at when you begin to troubleshoot it? Chances are, you're going to take a look at the file's extension, such as .avi, .mov, .mp4, .wmv, .ogv, or others.
This is the root of a lot of people's misunderstanding of video files and one of the reasons that figuring out a video file can be so confusing: there are actually multiple formats associated with any video file, and only looking at the extension, which indicates the container format, is an oversimplification (within this container, you can find video and audio tracks that are formatted in different codecs).
The container format can be somewhat informative, and there are both problems and advantages to different container formats, but in general, the audio and video codecs used are more informative. In a way, the container format is a lot like a .zip or .rar file: it does have its own unique qualities, but mostly, it's a package that holds other files together.
What container format should you use? It really depends a lot on your workflow and what software you're using to export videos. As we note in our documentation on exporting video, we generally recommend .mp4 or .mov format for maximum compatibility.