A staple is a wire bent into a U shape that is applied with an airgun. It is a very quick solution to fasten not too thick wood, with less precision and aesthetic quality.
When it is used without glue it is not a very durable connection, but it can be used as a temporary solution. Small wooden crates are often stapled together. Another comon use of staples is for connecting fabric to furniture in upholstering.
Examples on Superuse.org
Examples on Harvestmap.org