What is the basic work for solid state relay and what applications are used?

Is an electronic switching device that switches on or off when a small external voltage is applied across its control terminals. 

Temperature Controller 

AC DC Switching 

IN PLC Device, Power supply, Modules

Ussually it is posivite and negative 24dvc supply voltage ( or low voltage requirements),with normally close and normally open switching contact, triggering by thermocouple sensor to activate the heating system, and it is depends upon the setting value of thermocontroller. 

relays for high current application, usually used for heaters

SSR ARE MAINLY USED IN FAST SWITCHING & PRECISE APPLICATIONS

SUCH AS TEMPERATURE CONTROLS FOR HEATER ON/OFF SWITCHING USING PID CONTROLS

BASIC WORK OF SSR IS LIKE SIMPLE SWITCH OPEN & CLOSE THE CONTACT

Two major parts.

The relay is same like electromechanical relay but it has no moving parts it is basically normal open semiconductors which is designed to switch either Ac or Dc to the load and are used in applications where circuit under control must be protected from the introduction of electrical noises

for high speed opening and closing loads, perfect device. for exemple heaters, selenoid valves... i use them all systems instead of the contactor.

to do the changeover process, solid relay play a vital role.

Solid State Relays are normally-open semiconductor equivalents of the electromechanical relay that can be used to control electrical loads without the use of moving parts.

Application of solid-state relays must consider their lower ability to withstand momentary overload, compared with electromechanical contacts, and their higher "on" state resistance. Unlike an electromechanical relay, a solid-state relay provides only limited switching arrangements.

In general, relays are used in applications where you need to isolate the signal source from the load for protection, safety or control purposes, for instance, you can use a relay to activate/energize a high power circuit/load (a motor) using a low power source (a PLC). 

Solid State Relays (SSR's) are electronic relays (there is no direct connection between the source/coil and the output/switch). Instead of using magnetic mechanisms, SSR's use thyristors or transistors as coupling components.

SSR's are best used in applications where you need a faster switching response, and/or you have more frequent switching events as the lifetime of the SSR is much longer than a conventional relay (electromagnetic)  

The solid state relay may be designed to switch either AC or DC to the load. It serves the same function as an electromechanical relay, but has no moving parts

The most common application of solid state relays  is in the switching of an AC load, whether that is to control the AC power for ON/OFF switching, light dimming, motor speed control or other such applications where power control is needed.