A tilde is a diacritic mark (~) put over a letter (usually a vowel) to indicate nasalization. For example, in Portuguese, ã and õ are nasalized a and o. In Spanish, tilde over n (ñ) is a separate letter (called eñe) and is a palatal [n] (SAMPA J, IPA [ɲ]), pronounced like nh in Portuguese.
In the International Phonetic Alphabet (IPA), the tilde is used to mark nasalization, and is placed above any phone that is nasalized.
In Japanese, this symbol is used to indicate ranges. 12 ~ 15 means "12 to 15", ~ 3 means "up to three" and 100 ~ means "100 and greater".
Used in URLs on the World Wide Web, it often denotes a personal web page or web site which resides on the website of another company or organization. For example http://www.widgets.com/~johndoe/ might be the personal web site of John Doe, on the website of the Widgets company. This comes from the Unix shell usage of ~ followed by a user login to mean the user's home directory, but in the web often means a directory called public-html inside the user's home directory.