A pocket hole joint is a connection point in a piece of carpentry where two surfaces are united with screws inside sunken holes. Pocket joints make an elegant solution for connecting elements securely—they're easy to tighten, and they don't require any complex math to create.
A pocket hole jig can make drilling these pockets much faster and easier, since it can attach to the wood while working, creating more unified and even holes. Jigs can be large pieces of equipment to which wood is secured, or portable devices that clamp on where they're needed.
Pocket screws make the best connectors for these kinds of joints. They're a little pricier than normal screws, but they're self tapping, which means they can create their own hole as they screw in, and they have a slightly wider head, to prevent cracking through the wood at the base of the hole. For soft wood, use coarser threads (wider apart)—for hard wood, use finer (closer together).
Because of their angle (typically about 15 degrees), pocket hole joints are stronger than other traditional joining methods. They still require a perpendicular connection between the two elements for maximum strength and durability. Most are screwed into a board's edge or face instead of the end grain for maximum purchase.
The only downside of a pocket joint is that because it grabs the wood so effectively, it only fails when the wood itself breaks. Repairs at that point require replacing at least part of the project.