In CSS IDs are similar to classes, except once a specific id has been declared it cannot be used again within the same (X)HTML file.
I generally use IDs to style the layout elements of a page that will only be needed once, whereas I use classes to style text and such that may be declared multiple times.
The main container for this page is defined by the following.
Everything within my document is inside this division.
I have chosen the id selector for the “container” division over a class, because I only need to use it one time within this file.
Then in my CSS file I have the following:
border: 1px solid #666;
You will notice that the id selector begins with a (#) number sign instead of a (.) period, as the class selector does.