Pocket screw

Craig Sommerfeld invented pocket joinery back in 1986, and the company he started – Kreg Tool — has been continually refining the technology ever since.

Kreg Pocket Hole Jig


What is pocket hole joinery:
Drill an angled counter bore into a piece of wood and joining it to another piece of wood with a screw. The bottom of the counter bore (the "pocket hole") sits about 1/2' back from the edge being joined and provides room for the head of the screw. The clearance hole aims the screw at a sharp angle so that it can take a solid hold the adjoining material without popping up through the surface of the wood.

Strength:
Independent testing found that a pocket screw joint failed at 707 pounds when subjected to a shear load while a comparable mortise and tenon joint failed at 453 pounds - meaning that the pocket screw joint was approximately 35% stronger.

Application:

Cabinet frames, face frames and carcasses are among the most popular applications for pocket hole joinery.
Edge-joining (without bar clamps). 
Leg Rails and Braces
Stairs
Edging
Curves
Decks 

Pocket hole without a jig is also possible: 
https://www.woodmagazine.com/woodworking-tips/techniques/joinery/pocket-hole-joinery

This type of assembly method is widely used on mass produce furniture. Glue is not necessary since the screws act as internal clamps holding the joint together, making it possible to take down and put together afterwards. It's relatively strong with the help of some glue, and self tapping screws. There's no need for aligning two holes as is required with dowels and mortise and tenon joints. Apart from the fact that this method makes the joinery not quite pleasing to the eyes, its definitely a widely used method and a pretty effective one.