We use language to accomplish certain kinds of acts, broadly known as speech acts. These acts are distinct from physical acts like drinking a glass of water. And they are distinct from mental acts like thinking about drinking a glass of water. Speech acts include asking for a glass of water, promising to drink a glass of water, threatening to drink a glass of water, ordering someone to drink a glass of water, etc.
When we talk about speech acts (according to Austin), a distinction is drawn between direct and indirect speech acts. Of the direct speech acts, there are posited to be 3 basic types. These 3 basic types correspond to 3 special syntactic types in most of the world's languages and convey (i) assertions, (ii) questions, and (iii) orders/requests. Examples are given below in English, French and Buang (a Malayo-Polynesian language of Papua New Guinea).
The examples in (1) are all assertions. Assertions use the declarative form. The function is to convey information. Assertions are either true or false.
(1) Assertions
(a) Jenny got an A on the test.
(b) Les filles ont pris des photos. ('The girls took photos')
(c) Biak eko nos. ('Biak took the food')
The examples in (2) are all questions. They use the interrogative form and their function is to elicit information. They don't have a truth conditional status.
(2) Questions
(a) Did Jenny get an A on the test?
(b) Les filles ont-elles pris des photos? ('Did the girls take photos')
(c) Biak eko nos me? ('Did Biak take the food')
Lastly, orders and requests use the imperative form. Their function is to cause others to behave in certain ways. Examples are shown in (3).
(3) Orders
(a)Get an A on the test!
(b) Prenez des photos! ('Take some photos!')
(c) Goko nos! ('Take the food!')
Although assertions, questions and orders are fairly universal, and most of the world's languages have separate syntactic constructions to distinguish them, there are also other speech acts. Those other speech acts do not have a syntactic construction that is specific to them. Example (4) contains an if-then construction. The if-then construction isn't reserved for conveying a particular meaning but here it is being used to convey a warning or threat.
(4) If you cross that line, then I'll shoot you! 
Example (4) shows one way of conveying a threat, but it's not the case that if-then constructions only convey threats or that threats are always conveyed via if-then statements. A threat has no special syntactic form.
Given the diversity of expressions that are possible and the variety of interpretations that can be assigned to particular constructions, this raises the point that what one *does* things with language and what one does depends on some contextual factors. To start, consider (5) and imagine a context in which Joe is an on-duty police officer.
(5) You are under arrest. [Joe to Mary yesterday]
If I now ask what Joe did yesterday, is it correct to say that Joe said that Mary was under arrest? No, because what's important is not that he said those words. What happened yesterday is that Joe arrested Mary. With those words. Here's another example of someone *doing* something with language (i.e., changing the state of the world):
(6) The court is in session. [the Judge at 9am]
If you heard the example in (6) and were asked later what the judge did at 9am, it's not that the judge SAID that the court was in session. Rather, what happened was that the judge opened the session of court. He made the status of the court session undergo a change of state simply by uttering the words in (6) (and by being a person who meets the prerequisite conditions to say such words and have them take effect).
When we talk about utterance meaning, we have to acknowledge that many utterances are not used to make statements that can be assessed as true or false. Some of those include non-declaratives such those in (7-9), which convey questions, commands, requests, threats, etc.
(7) What's the time?
Who's coming to the party?
Do you want cream in your coffee?
(8) Get out!
Tell me your name.
Don't talk to strangers.
(9) Please help me with my homework.
Feel free to drop in.
Do that and she'll dump you.
But we also have many declaratives that do something other than make a statement which can be evaluated as true or false. See (10). None of the sentences in (10) are true or false. Rather their propositional content becomes true by uttering them (if the person uttering them and the specifics of the context meet the prerequisite conditions).
(10) I hereby declare you man and wife.
I bet you ten pounds I'll win.
I apologise.
I sentence you to five years in jail.
Utterances like those in (10) have some special properties. Notably:
Verbs that typically appear in utterances of this type include "declare", "sentence", "christen", "apologise", "promise". These are all examples of performative verbs and the utterances in which they occur are called performative speech acts. The alternative (the regular statements that we encounter) are called constantives (e.g., "It's raining now") which can be assessed as true or false. What's interesting about performative verbs is that the action of the utterance is inherent in the word. Performatives have no truth value, just felicity conditions that govern their use. This is the sense of "language as action". Constatives on the other hand are assertions with true values; their use doesn't constitute action.
KEY POINT: Language can be understood as action. We commit acts by speaking. It's how we change the world through words.
To go on to section 6.2 "Performatives", click here.