Conservation of electric charge
Also called Charge conservation
Conservation of electric charge is the rule that a system's net charge stays constant unless charge is transferred to or from that system. Charging by friction, contact or grounding moves electrons between objects; none of them creates or destroys charge.
Both AP Physics 2 and AP Physics C: E&M name a whole topic after this, and both state it as a pair of sentences rather than an equation. EK 10.2.A.2 in AP Physics 2, matched by EK 8.2.A.2 in C: E&M, says any change to a system's net charge is due to a transfer of charge between the system and its surroundings, and EK 10.2.A.2.ii adds that the net charge of a system will be constant unless there is a transfer of charge to or from the system.
Draw the system boundary first. The rule is about a system, so the answer to "is charge conserved here?" depends on where you put the line. Rub a balloon on hair and the balloon's charge changes; draw the boundary around balloon plus hair and the total has not moved at all.
What actually travels. EK 10.2.A.2.i says charging typically involves the transfer of electrons to and from the system. Protons stay put in ordinary matter, so a positively charged object is one short of electrons rather than one loaded with protons.
Using it as bookkeeping. Two conductors brought into contact redistribute charge until their surfaces sit at the same potential (EK 10.2.A.1 in C: E&M), and the total they started with is the total they end with. For two identical spheres, equal potential at equal radius means an even split.
There is no equation for this on any AP sheet. It is a constraint you impose on a problem, the way you impose momentum conservation, and it holds at every step of charging by induction.