A shot is is pretty basic, it outlines each and every shot you are going to take in your film.
This tutorial by Web Video Crew does a good job of covering the basics of creating a shot list, and will only take 2 minutes of your time.
Creating a shot list is as simple as looking at your script, and thinking: "When Jimmy picks up the coke and drinks it, i want that to be a medium shot" Rinse and repeat for the whole script.
What you'll end up with is a line by line resource of every shot in your movie. This is invaluable to help you plan each shoot and stay organized and focused. Once you have your shot list, which includes all B-Roll(shots which don't necessarily require actors, such as an establishing shot of a building, a tree, a glass of water, etc...) And you can break the script down(Called a script breakdown) to organize how you're going to shoot. Having this information organized helps you see what schedule of shooting will work best for each day/scene/situation.
Shot List vs Storyboard
OK, so both are traditional filmmaking tools. The big boys usually use both, indie films can sometimes choose one or the other. If i were to pick just one, it would be the shot list, mainly because it can be used in more ways than a storyboard. Storyboards are great for being able to show everyone involved the look that you're going for, which is great. But if for whatever reason you reason you only want to do one or the other, i'd pick shot list, it can be used to better effect for organization.