What does it matter wheater I use:
And HTML:

Or I use:
And then HTML:

Could somebody explain please?

Thank You.

The difference is that an ID references a unique element on the page...i.e. there is only one element on the whole page with the ID.

A class can be used to name several different elements on the page.

In other words, you can have one div with a specific ID, but multiple divs with a specified class.

In your case, you would probably want to use a class.