A capacitor is simply two pieces of conducting material insulated from each other.
Normally, if one of the conductors is charged positively, the other is charged
with an equal amount of negative charge. The positive conductor will be at a
higher voltage than the negative conductor, and the charge
is proportional to the voltage difference between them.
In this equation, and in all of the circuit relations we will use this semester, Q and V indicate magnitudes only. Note also that V is always the voltage difference. You will have to correctly handle negative signs yourself by drawing pictures and thinking carefully. The word capacitance refers to the capacity of the system to store charge. For the same voltage, a pair of conductors with a large value of C has a larger capacity to store charge than a pair with a small value of C.