ParagraphIndent is an option for Cell which specifies how far in printer's points to indent the first line of each paragraph of text.
A new paragraph is taken to start at the beginning of a cell, and after every explicit Return character in your text.
Negative settings for ParagraphIndent make the first line of each paragraph stick out to the left.
See The Mathematica Book: Section 2.10.10.
See also: AutoIndent, LineIndent, ParagraphSpacing.