What is the difference between an 'id' selectors and 'class' selectors?

Question

Consider these:

<section id = "paragraph1">
<p>This is the first paragraph</p>
</section>

// or

<section class= "paragraph1">
<p>This is the first paragraph</p>
</section>

// I'm confused, DOM is tricky.

Answer 1

Well an id selector is only supposed to be used once for a single element but a class selector can be used mutiple times. Example:

<div class="partOne" id="sectionOne">
</div>
<div class="partOne" id="sectionTwo">
</div>
<div class="partOne" id="sectionThree">
</div>

And then in JavaScript they are called in two different ways:

For individual elements with assigned IDs:

document.getElementById("sectionOne");

and for groups of elements with an assigned class:

document.getElementsByClassName("partOne");