One where the thread goes all the way down.
A full set of 3 taps same diameter the first is a leader tap tapered to start the thread.
Second has less taper to thread deeper and the third has no taper thread to the bottom.
Only applicable on shallow holes that need threading to bottom out.
If your hole goes all the way the material a plug tap (second one) will suffice, this is whats usually supplied with the cheaper sets that have only one tap of each size.
There will be arguments but thus is a laymen's explanation.