What the section sign means
The § sign stands for "section". It is used mostly in law to point to a numbered section of a statute, regulation or contract: 17 U.S.C. § 107 is section 107 of title 17 of the United States Code. Doubled, §§ means "sections", as in §§ 101–105. Legal citation style puts a space after the sign, and a non-breaking space keeps § and its number on the same line.
In Germany, Austria and other countries whose laws are divided into numbered paragraphs, § is read as "Paragraph" and is so common that it is printed on the 3 key of German keyboards.
Where the § sign comes from
The usual explanation is that § began as two interlocking letters S, short for the Latin signum sectionis, "sign of the section". In traditional typesetting, § is also the fourth footnote mark, used after the asterisk, the dagger † and the double dagger ‡ have been taken on a page.
How to type the section symbol
On a Mac with a US keyboard, press Option + 6; Mac keyboards sold in the UK and Europe often have a § key in the top-left corner. On Windows, hold Alt and type 0167 on the number pad (Alt + 21 also works in many programs). On German keyboards, it is Shift + 3.
In Word, type 00A7 and press Alt + X. In HTML, use §, and in LaTeX \S. On iPhone, you can add a text replacement under Settings > General > Keyboard > Text Replacement that turns a short code such as "sss" into §.
Citing paragraphs instead? Use the paragraph symbol ¶. For copyright notices and trademarks, see the copyright symbol and trademark symbol pages.