''In certain situations, then, we vocalise in order to send messages through the air to other members of our species. Such situations are speech events.''

''In the approach to the analysis of speech events termed speech act theory, the message sent, the content of the communication, is a form of human action. This action is not the act of speaking, but an act we perform by speaking -- a speech act.''

from: http://www.arts.uwa.edu.au/LingWWW/LIN101-102/NOTES-101/speech_acts.html
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''...speaking a language is performing speech acts, acts such as making statements, giving commands, asking questions, making promises, and so on. - JohnSearle''

Also check: http://www.rdillman.com/HFCL/TUTOR/Relation/relate2.html

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''Speech Acts (SA) are utterances which contain information needed to assert and perform actions, or, according to Austin "things that people do with words". Speech Act Verbs are verbs used in speech acts utterances, to perform actions.''

from: http://web.archive.org/web/20050404152835/http://www.arch.usyd.edu.au/~anna/papers/caadf97.html
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To understand how TerryWinograd and FernandoFlores used it to model a conversation:
http://hci.stanford.edu/cs147/readings/language-action.htm