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I've heard all these terms thrown about in proofs and in geometry, but what are the differences and relationships between them? Examples would be awesome! :)

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Go read this Wikipedia article and the articles it links to. –  kahen Oct 24 '10 at 20:22
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One difficulty is that, for historical reasons, various results have a specific term attached (Parallel postulate, Zorn's lemma, Riemann hypothesis, Collatz conjecture, Axiom of determinacy). These do not always agree with the the usual usage of the words. Also, some theorems have unique names, for example Hilbert's Nullstellensatz. Since the German word there incorporates "satz", which means "theorem", it is not typical to call this the "Nullstellensatz theorem". These things make it harder to pick up the general usage. –  Carl Mummert Oct 24 '10 at 23:15
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In Geometry, "Axiom" and "Postulate" are essentially interchangeable. In antiquity, they referred to propositions that were "obviously true" and only had to be stated, and not proven. In modern mathematics there is no longer an assumption that axioms are "obviously true". Axioms are merely 'background' assumptions we make. The best analogy I know is that axioms are the "rules of the game". In Euclid's Geometry, the main axioms/postulates are: (I) Given any two distinct points, there is a line that contains them. (II) Any line segment can be extended to an infinite line. (III) Given a point and a radius, there is a circle with center in that point and that radius. (IV) All right angles are equal to one another. (V) If a straight line falling on two straight lines makes the interior angles on the same side less than two right angles, the two straight lines, if produced indefinitely, meet on that side on which are the angles less than the two right angles. (The parallel postulate).

A theorem is a logical consequence of the axioms. In Geometry, the "propositions" are all theorems: they are derived using the axioms and the valid rules. A "Corollary" is a theorem that is usually considered an "easy consequence" of another theorem. What is or is not a corollary is entirely subjective. Sometimes what an author thinks is a 'corollary' is deemed more important than the corresponding theorem. (The same goes for "Lemma"s, which are theorems that are considered auxiliary to proving some other, more important in the view of the author, theorem).

A "hypothesis" is an assumption made. For example, "If $x$ is an even integer, then $x^2$ is an even integer" I am not asserting that $x^2$ is even or odd; I am asserting that if something happens (namely, if $x$ happens to be an even integer) then something else will also happen. Here, "$x$ is an even integer" is the hypothesis being made to prove it.

See the Wikipedia pages on axiom, theorem, and corollary. The first two have many examples.

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Arturo, I hope you don't mind if I edged your already excellent answer a little bit nearer to perfection. –  J. M. Oct 25 '10 at 0:26
    
@J.M.: Heh. Not at all; thanks for the corrections! You did miss the single quotation mark after "propositions" in the second paragraph, though. (-: –  Arturo Magidin Oct 25 '10 at 0:33
    
Great answer. Clear and informal, while still accurate. Better than wikipedia's, in my opinion. –  7hi4g0 Feb 8 at 0:47
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