Resistors only allow limited amounts of current to flow through them. A resistor is indicated by a rectangle drawn parallel to the circuit. The wire line doesn’t go through the rectangle.
rheostat variable resistor also limits current, but its upper limit is variable. Lamp dimmers are variable resistors. This is indicated by a resistor rectangle with an arrow drawn through it.
Some variable resistors are controlled by potentiometers, which are used to adjust voltage. This is indicated by a resistor rectangle with a downward-pointing arrow on top of it.
A preset variable resistor is cheap style of variable resistor that is adjusted with a screw and screwdriver. This is indicated by a resistor rectangle with a capital T drawn through it.