The most common and easiest type of centering is that of lines of text in an element. CSS has the rule text-align: center for this purpose:

HTML

<p>Lorem ipsum</p>

CSS

p {
    text-align: center;
}

This does not work for centering entire block elements. text-align controls only alignment of inline content like text in its parent block element.

See more about text-align in Typography section.