Electric symbols

When drawing wiring schematics and wiring diagrams, We need a simple language to determine the function of parts used in HVAC equipment.  This language is symbols.  I would like to say that this language is universal.  It is not.  The symbols used here are the most common I have seen.  All component symbols are shown in the deenergized or power off position. Normally off means the switch is open.  No current will flow.  
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Switches

A switch can be either normally closed or normally open.  The switch on the left is normally closed.   The switch on the right is normally open.  Both of these switches are single pole single throw switches.  The single pole means  it will pass power thru a single circuit.  Throw means the action of the switch.  

Usually there is an indication of what actuates the switch.  The above switches show no method of activation and are assumed to be manually operated switches.  

The switch above is also manually actuated but it is 2 single pole single throw normally open switch but if one switch is closed, the other is connected and closes.  

Thermostat

The symbols below indicate the switch is operated by temperature.  On the left the thermostat is a heating one.  It opens on rise of temperature.  On the right is a cooling thermostat.  It opens on a drop in temperature.  

Below we have 2 more thermostats.  The difference here is they are normally closed.  These are usually limit switches.  They will be closed until the temperature reaches a point that the circuit must be opened.  The one on the left is an open on drop in temperature.  The one on the right is an open on rise in temperature.  

Pressure switches

The switches below are activated by pressure.  The one on the left is normally closed open on rise in pressure.  On the right is one that is normally closed open in drop in pressure.

Timers

The switch below is a normally closed time actuated switch.  

Below is another time switch.  This one is a single pole double throw switch.  This switch is neither open nor closed.  Rather it is closed to one circuit when the unit is off and open to the other circuit.  When the right amount of time has passed in the case of this switch, the action moves from the normal position to the energized position.  This type of switch can be actuated by pressure, temperature and any other method.  

Contactors

Below are several types of contactors.  A contactor is is an electrically actuated switch.  On the left is a single pole single single throw normally open switch.  The center is a double pole single throw normally open switch.  On the right is a triple pole single throw normally open switch.  

Relays

Relays are also electrically actuated switches.  The difference between a contactor and a relay is generally relays control amperage of 20 amps or less.  They also are commonly normally open or closed whereas contactors are mostly normally open.  The relay below is a single pole double throw.  

Below is a relay showing both the contacts and the coil that energizes the the relay to change its position.  In Schematic diagrams, the coil and the contacts are usually in different places.  This is also true of contactors.  

Motors

Motor symbols can vary quite a bit.  Below left is a simple representation of a motor that could be a shaded pole motor.  The center is a multi speed motor.  On the right is a PSC motor with its capacitor.  

Below is a symbol for a humidistat.

Below is a symbol for a light

Below is the symbol for an electric heater.  

Below is the symbol for a transformer.

Below is a symbol for a normally closed overload.

There are other symbols, some used by some manufacturers in their diagrams that are somewhat different.  There is not an effective standardization of symbols.

Below is a video thats shows some of the symbols and explains.

When reading a diagram, somewhere on the diagram will be a key to the symbols used.  As I have said before, not all symbols are the same throughout the industry.  
The key below shows the symbols used in some diagrams.