This is example 1. This paragraph of text has an indent of 3 ems. Because its value is positive (greater than 0), all the text stays within the element. Things get more complicated if I start using negative margins to make the first line "outdent" instead.
This is example 2. This paragraph of text has a negative indent. Because this element does not have enough space to its left, the hanging text is clipped by the edge of the browser window. The border of this element is displayed to help you see the nature of the problem.
This is example 3. This paragraph of text has the same negative indent as the example above, but because this element has a left margin value greater than the negative indent value, the hanging text is not clipped by the edge of the browser window. The border of this element is displayed to help you see how a potential problem is avoided.
This is example 4. Here is the same styling as the previous paragraph, but without the element border so you can clearly see the effect. Indents provide a visual indication of the start of paragraphs when you don't want to use space to separate them.
Here is a second paragraph with no space between it and the previous one. The negative value indent makes it clear that this is a new paragraph. Positive value indents can serve the same purpose.
A code example from Stylin’ with CSS, Third Edition by Charles Wyke-Smith. Visit stylinwithcss.com for more CSS information and updates.