Soldering is the metallurgic process used to fix electronic components to a PCB, usually with a tin alloy. Soldering can be done by hand or automatic. PTH boards were usually wave soldered, SMT uses reflow technology.
Further reading:
Videos:
- How to Solder: Part 1 by EEVblog
- How to solder: Part 2
- How to solder: Part 3 Surface mount components