Visibility

Function and state variables have to declare whether they are accessible by other contracts.

• external - External functions are meant to be called by other contracts. They cannot be used for internal calls. To call an external function within contract this.function_name() call is required. State variables cannot be marked as external. 

        --- Only other contracts and accounts can call

• public - Public function/ Variables can be used both externally and internally. For public state variables, Solidity automatically creates a getter function.

       --- any contract and account can call

• internal - Internal function/ Variables can only be used internally or by derived contracts.

    --- only inside the contract that inherits as an internal function.

• private - Private function/ Variables can only be used internally and not even by derived contracts.

Example:-

pragma solidity ^0.8.0;

contract Test{

function A() external pure returns(string memory) {
return "It's me Anurag";
    }
}

contract Test2{

// contract instance
Test test = new Test();

// public - if you deploy it they will show the return value and in public, anyone can call it.
function B() public view returns(string memory) {
return test.A();
   }

// private - private keyword don't show the return value.
function C() private view returns(string memory) {
return test.A();
    }

// internal - only connect with internal or derived contracts.
function D() internal view returns(string memory) {
return test.A();
  }

//external - only call other contract.
function E() external view returns(string memory) {
return test.A();
 }

}