What is a drop bag?
A drop bag is some sort of container used to hold supplies needed during a race. The drop bag is dropped at some point along the race route. You will have access to the bag when you reach that part of the course.
Is a drop bag actually a Bag?
Not necessarily, though that is the most common container. Some people use backpacks, duffel bags, plastic shopping bags, or garbage bags. Other containers can be used, too. I like Rubbermaid containers. Some people use five gallon buckets. More or less any container will work, though it should be water-tight. Each race may have specific rules that dictate size, shape, and acceptable containers.
Do you NEED drop bags?
If you have a crew and that crew has access to every aid station where drop bags are allowed, then no. If your crew does not have access to some drop bag locations, they can be useful.
What goes IN the drop bags?
Drop bags can be used to stash almost any supplies needed for the race. Common contents include things like food, gels (technically food... but barely), electrolytes, first aid crap, lube, dry socks, spare clothes, rain gear, sun screen, insect repellent, anti-diarrhea meds, batteries, or whatever else you think you need.
How do you know if you pack too much?
For my first ultra, I had about 12 five-quart Rubbermaid containers full of assorted "goodies." Each drop "bag' was packed with everything I thought I would need. That turned out to be a huge mistake. When I arrived at the aid station, it took way too long to dig through the container to find what I needed. I had a ton of crap that just got in the way. So how do you determine what is needed? Make a list of everything you think you need. Figure out what items will be available at the aid station and remove them from your list. Now eliminate unnecessary items (no need for batteries at an aid station you'll pass at noon.) Now put everything else in the drop bag. Go to an open field and throw it as far as you can. If you can't throw it more than 50 feet, you packed too much.
Experienced ultrarunners- what goes in your drop bags?