Drums come in different sizes but a typical size is 55 gallons.
The top of the drum may be closed or open. A good brief video showing the difference is here: https://www.youtube.com/watch?time_continue=73&v=GxuyD1e5WT0
-Open head drums are better for solids and thick liquids. The entire top of the drum comes off. The lid may be solid or have bung holes in it to hold a pump. They are easier to empty and clean out, but they aren't leak proof.
-Closed head drums are sealed except for one or more holes. These holes are typically threaded to accommodate pumps. The tight seals make them leakproof. These are better for liquids, especially those you want to avoid spilling such as hazardous liquids.
Different materials and ratings make the drums appropriate for different things, ranging from acids and caustic liquids to thick liquids and solid and granular materials.