is a generic term
for electrical apparatus the controls the flow of power in a building. Switchgear includes panelboards
and motor starters
. But it does not include a light switch
, which is considered a device
in the National Electric Code
A panelboard is a piece of electrical equipment capable of disconnecting more than one circuit, and each cicuit has it's own overcurrent protection be it a fuse or circuit breaker. Essentially, your house electrical panel is a very small panelboard, but they can get really large in commercial and industrial applications.
A disconnect is an electrical apparatus capable of disconnecting all electrical power from a single piece of equipment. Like a transformer or motor. A disconnect may or may not offer some type of overcurrent protection.
The kind of swtiches you use to turn off your lights may or may not be classified as a disconnect. Most commercial grade switches are motor rated, at least for motors under one horsepower. The fart fan in your bathroom is powered by such a motor.
Disconnects may be used to shut a piece of equipment down, so it may be safely maintained. The circuit breaker in your panelboard may serve as a means of disconnect, but according to the NEC it must be located with 30 feet (ten meters) of the equipment to be disconnected, and must be in your line of sight. A lamp cord may also serve as a means of disconnect, but it is itself not a disconnect.
In your home, or any other building, there must be a means of disconnecting all electricity at the service entrance. For many people, the main breaker in your panel serves as said disconnect. However, the main breaker may not be used as a disconnect unless the panel is within 3 feet (one meter) of where the electrical service enters the building (It can be farther if the service conductors are underground and encased in concrete. In which case a disconnect shall be set outside the premises. For most homes, that's simply a breaker-in-a-box, or a main breaker in it's own, weather rated enclosure. At my house, I set a fused disconnect, which looks a lot cooler.
A Motor starter is a piece of equipment capable of starting, stopping, and potentially reversing an electric motor. Invariably a starter is also capable serving as a disconnect. It will always offer at least one type of overcurrent protection, usually fuses and thermal relays, so the motor is protected for both current and temprature. This protection is sized less for dead shorts than to protect the motor from overloads that might damage it.\
Another important difference is that a motor starter of any size has a set of magnetically controlled contacts that are used to switch the motor on and off. That allows for the motor to be manually controlled, or to operate and shut down automatically. For example, if you had a large water tank, you might use two limit switches to keep the tank topped off. One low water switch to start a pump used to fill the tank, and a high water switch to shut down the pump when the tank is full. In a fire pump, a small jockey pump controlled by pressure switches is used to keep the sprinkler systems up to pressure.
Motor starters may be grouped together, in what amounts to a very large panelboard known as a motor control center. The starter can be at any location, as long it is disconnected according to the thirty foot/ line-of-sight rule required by the NEC.
Switchgear can be almost any size, starting with a small 30 ampere four breaker panel sold in most hardware store, to 10,000 amp, high voltage gear found in industrial applications.