An exclamation mark is an ugly word that is used in sentences such like "I'm a transvestite!" or "**** you son of a *****!", or maybe even "I mowed over my cat with the lawnmower!", and it expresses different moods, such as horror (the transvestite thing), anger (the son of a ***** thing) or excitedness (the cat and the lawnmower thing).
It is known that the first exclamation mark was used in Egyptian times when two Egyptians were chatting on the Internet:
dude 1: "the end of the world is now!"
dude 2: "wtf did u just use that "!" for? are u a douche or something?"
dude 1: "oh shut up, ur just jealous that i came up wit somethin u were too stupid to think of."
dude 2: "im not jealous of anything, u douche."
dude 1: "is that all u can think of to say to me? douche? wtf is a douche?"
dude 2: "idk. something related to girls i think. who cares? does it even matter? ur a dork anyway."
dude 1: "like, who even uses the word 'dork' anymore? are u retarded or somethin?"
dude 2: "just shut up. im blocking u."
And then he did, and how this conversation was found was because one of the Egyptian dude's girlfriends read it and decided to publish it on Wikipedia and delete an entire article (the main page) and so that was what happened, and soon everyone started reading that article when they logged onto Wikipedia and discovered that it was actually interesting.