Capacitors store electric charge. A basic capacitor is indicated by two lines of equal length drawn perpendicular to the wire line, with only blank space between them.
Some capacitors are polarized, and one end must be close to the positive terminal of the power supply. This is indicated by two rectangles drawn perpendicular to the wire line, with blank space in between. The rectangle on the negative side is shaded in; the positive isn’t.
Variable capacitors store varying amounts of electric charge. A variable capacitor is indicated by an arrow drawn through the capacitor symbol.
A trimmer capacitor is like a present variable resistor: It’s cheap to make and operated with a screw and screwdriver. It’s indicated by a capacitor symbol with a T drawn through it.