Let's begin with the most basic smart contract example,  HelloWorld.sol, shown as follows:

pragma solidity ^0.4.24;

contract HelloWorld {
  string public greeting;

  constructor() public {
    greeting = 'Hello World';
  }

  function setNewGreeting (string _newGreeting) public {
    greeting = _newGreeting;
  }
}

Solidity's file extension is .sol.  It is similar to .js for JavaScript files, and .html for HTML templates.