An exclamation mark (or exclamation point) is a punctuation mark or, more pedantically, a tone mark. Like the full stop (or period), it marks the end of a sentence that is either an actual exclamation, "Wow!", a command, "Stop!", or is intended to be astonishing in some way, "They were the footprints of a gigantic hound!"
In type-setting or printing (and therefore when spelling text out orally), the exclamation mark is called a screamer or bang.
Frequent use of the exclamation mark, also called the mark of admiration, is common in writing by teenagers and in advertising.
Some brands cleverly, but confusingly, contain an exclamation mark, e.g. "Yahoo".
In some languages, most notably Spanish, a sentence ending in an exclamation mark must also begin with an inverted exclamation mark, ¡.
Several computer languages use "!" for a variety of special meanings, most importantly, logical negation, e.g. A != B means "A is not equal to B." In this context, the exclamation is named the "bang" character. Other programmers call it a shriek, and in the BBC Basic programming language it is called a pling and is used to reference a 32-bit word.
In the Geek Code, "!" is used before a letter to denote that the geek stubbornly refuses to participate in the topic at hand.