What is the difference between a relay and a switch?

1) Directly operating with Switch some limitation of handling Voltages & Current  it is purely manual.

2) Relays can be operable in Auto ,can handle any high voltages with suitable Relay- Breaker of any Current.It can be operated by any tiny switches safely

A switch is manually controlled 

A relay has an electromagnet to pull the switch down by switch I mean the lever in the relay to make contact 

New semiconductor relays use triac or other electronic switches to separate the input to output this could be done with led and  photo cell

For instance - starting your car!

The key switch is small, maybe computer-controlled electronics.

It activates a big heavy-duty relay that runs a motor to start the engine.

This is a typical scenario.

A relay is basically a switch that is controlled by an electric or electronic circuit. In the circuit that you show, when the coil circuit is closed the power from the battery energizes the coil and closes the contacts in the circuit for the light bulb. That's the condition the circuit with the bulb is in now. The relay armature contacts move from the upper contacts (show as an isolated dot) to the lower dot, (called the normally open contacts. The armature moves the contact from the normally closed contacts to the normally open contacts, closing the light-bulb circuit. Relays are not necessarily used for high power circuits. I recall when I worked at NASA that we had what we called dry circuit applications. That meant that the current was very small. We had to use relays sealed in nitrogen with gold contacts to keep the contacts from oxidizing and causing unstable contact. 

The simple definition of a relay that I learned is that it uses low power to switch high power circuits.

A switch makes and breaks a connection to the load directly, like a room light in a home or office.

In a relay, a switch activated the relay contacts to close a circuit.

The relay, circuit and load may be far away from the operator like in an aircraft where the engineer in the cockpit activates a pump located in the tail of the aircraft, or in an atomic reactor where the operator activates a cooling pump located in the reaction chamber where very high radiation is present.

A relay is activated by applying a voltage.

The word "switch" can be applied to anything that is controlled to be either highly conducting or highly non conducting, but ordinarily it implies it is operated by hand.

Switch ..... manually is operate by human hand to do On and Off.

Relay....... operate electrically to do On and Off by itself after system is running.

A relay is an electromagnetically powered switch which is usually remote from the console switch on a circuit. It is typically used when there is a need to apply a much larger current to a device, such as starting a car engine or a boiler. They are also useful when there is a need to trigger multiple circuits from a single switch.  So it's kind if like passing a baton in a relay race.  Relays are also useful in that they can reduce the length of expensive high amperage wiring and other circuitry.