The pilcrow is what most people commonly refer to as the paragraph symbol, or ¶. This non-alphabetic symbol varies from typeface to typeface, but is most often found in that most basic form.

The symbol can be used as an indent for separate paragraphs, or to designate a new paragraph in one long piece of copy, as Eric Gill did in his 1930s book, An Essay On Typography.

The pilcrow was used in medieval times to mark a new train of thought, before the convention of using paragraphs was commonplace.

The pilcrow is commonly drawn like a backwards letter P with an extra full-height stem (¶), but may also be drawn with the round area stretching further downwards, more like a backwards D.

Etymology: Perhaps originated in the perversion of the word 'paragraph' through 'parcrafte', but this is not sure.

Additional Etymology: (q.v. http://www.typophile.com/forums/messages/29/1486.html )

"According to Parkes's Pause and Effect: An Introduction to the History of Punctuation in the West ... the pilcrow is a symbol for 'paraph' (which can also be marked by a double-slash, or a full-height cent-like sign), and it started as a 'C', for capitulum.
"BTW, Parkes also says that the pilcrow replaced the 'paragraphus' (which was marked in various ways, including a section symbol)."