What is the HTML tag for a background?

There is no HTML tag for a background as such. You put in backgrounds by using the attributes of certain tags, depending what you want to put a background on and how you want to display the background. You can also use styles to do it. You can have a colour or an image as a background. The bgcolor attribute is used in a lot of different tags to put background colours on things. So if you wanted the background colour of your page to be red, then you would have the Body tag with the bgcolor attribute like this:

If you wanted an image as the background to your page, you'd have:

Using styles you can do more complicated things with backgrounds. For example, you can use a style to put a background on a paragraph in different ways:

This paragraph will have your image as a background repeating horizontally.

Your paragraph text goes here

This paragraph will have your image as a background displaying only once.

Your paragraph text goes here

Those are just 2 ways, but there are lots more, and they can be applied not just to paragraphs but to other structures like the body or a table or whatever you want.