Synopsis:
\setlength{\parindent}{horizontal len}
\setlength{\parskip}{vertical len}
Both are rubber lengths (see Lengths). They affect the indentation of ordinary paragraphs, not paragraphs inside minipages (see minipage), and the vertical space between paragraphs, respectively.
For example, if this is put in the preamble:
\setlength{\parindent}{0em}
\setlength{\parskip}{1ex}
The document will have paragraphs that are not indented, but instead are vertically separated by about the height of a lowercase ‘x’.
In standard LaTeX documents, the default value for \parindent in one-column documents is 15pt when the default text size is 10pt, 17pt for 11pt, and 1.5em for 12pt. In two-column documents it is 1em. (These values are set before LaTeX calls \normalfont so em is derived from the default font, Computer Modern. If you use a different font then to set \parindent in units matching that font, do \AtBeginDocument{\setlength{\parindent}{1em}}.)
The default value for \parskip in LaTeX’s standard document styles is 0pt plus1pt.
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