whats the difference between class and id?

I plain terms in general with css, an ID has more specificity over a class.

e.g <div id="foo" class="bar">, styles specific to #foo will override .bar:

#foo { color: red; }
.bar { color: green; }

In general, it's best practice not to apply any styles to an ID and leave this for such things as javascript. Also, this can cause issues later on as ID's in some cases may need to be overridden using !important which can then bloat your css.

As the names imply, an ID is for a specific item, and a class is for a group of items.

So, if you want a group of items to appear the same, use class to define that look.

If you want just one specific item to have that look, you can use an ID.

When you work with JavaScript later on, you will find other uses for the ID, such as checking input, or adding output.

And as Max Weir has pointed out, the specificity also is different, the #id will override the .class directives.

They mostly do the same thing, but ids are supposed to be unique (appear only one time)