The acid core is mostly used in metal to metal and plumbing applications.
To iterate an important point is NOT to use any acid solder product in an electrical/electronic application. Electrolysis will be activated and corrosion will do you in.
A quick story of a new student in a military school...
All points about soldering are very informative and accurate. I would note that you do NOT use plumbing solder, only rosin core solder.
The mixture in solder is often tin to lead, which will help determine the melting temperature and shear strength of the solder joint. Usually you would use...