These are both timers in JavaScript. For some reason I always used to get these two confused, even though there's a clue in the name as to which does which.

setInterval

This is used to do something repeatedly after a certain amount of time.

setInterval(runFunction, 1000)

This will run the function called runFunction every 1000 milliseconds, ie 1 second. It will keep doing it until you tell it to stop.

To stop it you use clearInterval - but you have to have given the setInterval a name first.

let interval;

document.querySelector('.startButton').addEventListener('click', function() {
  interval = setInterval(runFunction, 1000)
})

document.querySelector('.stopButton').addEventListener('click', function() {
  clearInterval(interval)
})

function runFunction() {
  console.log('Running!')
}

This will print "Running!" to the console every second after you press the startButton, and stop once you press the stopButton.

You don't have to run a function from setInterval, you can use an anonymous function:

setInterval( function() {
  console.log('Running!')
}, 1000 )

Or with an arrow function:

setInterval( () => {
  console.log('Running!')
}, 1000 )

setTimeout

This is used to do something after a certain amount of time and then stop. So this will print "Running!" to the console once after 1 second:

setTimeout( () => {
  console.log('Running!')
}, 1000 )

Similarly, you can clear the interval afterwards:

const timeout = setTimeout(runFunction, 1000)

function runFunction() {
  console.log('Running!')
  clearTimeout(timeout)
}

Conclusion

setInterval and setTimeout are very similarly structured. The main difference is that setTimeout runs once after the timer times out and setInterval runs multiple times on the interval set.