The purpose of spot drilling is to drill a hole that will act as a guide for drilling the final hole. The hole is only drilled part way into the workpiece because it is only used to guide the beginning of the next drilling process. {https://en.wikipedia.org/wiki/Drilling}

Center drill bits, occasionally known as Slocombe drill bits, are used in metalworking to provide a starting hole for a larger-sized drill bit or to make a conical indentation in the end of a workpiece in which to mount a lathe center.
In either use, the name seems appropriate, as the bit is either establishing the center of a hole or making a conical hole for a lathe center.
However, the true purpose of a center drill bit is the latter task, while the former task is best done with a spotting drill bit. {https://en.wikipedia.org/wiki/Drill_bit#Center_and_spotting_...}

--------------------------------------------------
Note added at   1 godz. (2017-02-14 23:33:04 GMT)
--------------------------------------------------

Jeżeli chodzi o pytania:
1. nawiercanie (wykonanie lekkiego wgłębienia w celu lepszego prowadzenia wiertła. Potocznie nawiercanie oznacza też wiercenie otworu ślepego (nie na wylot).):
https://pl.wikipedia.org/wiki/Wiercenie
2. pre-drilling and pilot holes:
https://www.youtube.com/watch?v=nprmplSKqgA
https://www.youtube.com/watch?v=vUVJ9JXlZek
https://www.youtube.com/watch?v=2rEqVvW75Q4