Taps: Tools for Threading a hole during Construction

Tap: "a tool for cutting female (internal) screw threads" a Die is used to cut the male (outer) threads.

Taps are classified by diameter, threads per unit measure, taper and hand (manual) or machine (automatic/power) held.

Hand taps:

Plug tap (called a second tap in the UK): Tapered edges towards the end of the tap assist in aligning and starting the tap into a new hole. Flutes cut into the side of the tap allow the material removed to fall away.
Bottoming tap (called a plug tap in the UK): With no taper, this tap can cut threads right to the bottom of a hole without needing to protrude through the hole, but they are extremely difficult to start in a new hole
Taper tap: A very pronounced taper causes this tap to be less aggressive, removing only a small amount of material at each turn.

An excellent video on basic metal working.
The section on tapping starts at 5min, 55seconds.

Operation: When using a hand tap, the operator must insert the tap with very accurate alignment into the hole. The tap is then turned several times while pressing it into the opening of the hole to start the teeth cutting into the material. The tap must be turned back a quarter turn every turns in order to break the chip formed during the cutting process. If the chip is not broken, it can "crowed" or jam the tap, causing it to break. These chips need to then fall out of the flutes cut in the side of the tap. If the chips can not be cleared, the tap will have to be removed and cleaned, before re-inserting to continue the process.

Dry bar soap makes a wonderful lubricant for the tapping process. Because it is dry, the flakes of metal and soap easily fall away, preventing a buildup of material inside the hole. The cost is extremely low, and the cheaper soaps (e.g. Ivory) work best.

Machine taps:

Spiral point or gun tap: The cutting edge constantly breaks the chip of material being removed so it can fall out the flutes.
Spiral flute tap: Another type of spiral point tap, with flutes like a drill bit, to assist in removing the chips. This is especially important in "blind" (rather than through) holes, where chips can not fall through to the lower side.
Drap: Combination drill/tap: This tool saves time by both drilling and tapping in one process.

Calculate Drill Size for Tap Diameter, TPI, and Thread depth.