Answer:
The "end of proof" symbol or "halmos" is used by mathmeticians to mark the end of a mathematical proof. It appears as a solid (or sometimes open) box.
The halmos is called a "tombstone" by magazine writers who use it to mark the ends of magazine articles. It had been used by magazine editors and writers before it was adopted by mathmeticians.
See: http://home.att.net/~numericana/answer/symbol.htm#halmos
Unfortunately, it does not appear on the standard keyboard but is a part of the unicode and other character sets. It is often available (within some software programs) by typing special character sequences. Some writers have used // or [] as an attempt to replicate the box using only the standard keyboard characters.