A tilde (pronounced TILL-duh or TILL-day) looks like this: ~. It's a special typographic character found on most keyboards that means various things, depending on the context. In some operating systems, including UNIX, the tilde is used to represent the current user's home directory. On Web server systems, the tilde is frequently used by convention as the first character for any user's home directory in the file system. Since users often keep personal or business Web pages on a server under their personal home directory, you will often see the tilde as part of Web addresses. As a mathematical symbol, the tilde means "approximately" and in logic it means "not."
The tilde is one of the 128 alphanumeric and special characters in ASCII, the most common standard for electronic text exchange. The tilde happens to be ASCII character 126. It's sometimes called a "twiddle" or a "squiggle."