CSS: a language used for describing the presentation and design of an aspect of your site
To personalise the design of your site, you will use the design format Settings > Design > Personalize.
This is also where you can add CSS. This can be done in two ways, you will scroll to the bottom of the page where you will add or edit CSS (advanced users) or you can choose to put the stylesheet in expert - CSS mode. For more information, we recommend reading the tutorial : choose a theme and personalize the design.
A class and an ID: These attributes are used to apply your CSS styles to content. While there are few differences between classes and ids, the only one that is important is that ID can only be used once in a page. You can use ID's as anchors(a specific point in your page).
ID's are represented by a hashtag #. Classes are represented by a period .
For example
If I want to create a style for an image frame i would assign the attribute #imageframe
If I want to create a class for an image frame I would assign the attribute .imageframe
In this example, I would want to use the class .imageframe because I want to use this image frame more than once in my page.