There are two common types of flux cores: Acid and Rosin. Acid is for plumbing, and should NOT be used in electronics (it is likely to eat your components or boards). However, if you are looking for solder then you probably should pick up something with a rosin core.
Correspondingly, what type of soldering iron should I use for electronics?
Most of soldering irons used in the electronics are in range 20 – 60 Watts. Soldering iron with wattage 50W is very common these days and it will provide sufficient heat for most of soldering projects on the circuit boards. Soldering irons with higher wattage (40W -60W) are better.
Can you use acid core solder for electronics?
Rosin and acid core solders are manufactured with the solder wire as a tube and the center of the tube contains the flux. Rosin core solder contains a mild flux and is designed to solder parts where flux residue cannot be removed. An example would be electric circuit boards, or electrical connections.