Philosophical Distinctions

I. Epistemological Distinctions:

A Priori knowledge: knowledge obtainable without appeal to the senses.
(Ex: "Two apples plus two more makes four apples.")

A Posteriori knowledge: knowledge obtainable only through appeal to the senses. 
(Ex: "There are four apples in the basket.")

II. Semantic Distinctions:

Analytic truth: a sentence true simply because of the meanings of its words.
(Ex: "All bachelors are unmarried.")

Synthetic truth: a sentence whose truth is not due simply because of the meanings of its words.
(Ex: "A bachelor lives on 4th Street.")

III. Metaphysical (Modal) Distinctions:

Necessary truth: a true sentence which could not possibly be otherwise (i.e., could not be false).
(Ex: "Two plus two equals four.")

Contingent truth: a true sentence which could possibly have been otherwise (i.e., could possibly have been false).
(Ex: "There are four pieces of chalk in the room.")

Some Additional Modal Distinctions:

Essential Property: If it is a necessary truth that an object O has a property P, then P is an essential property of O.
(Ex: It is a necessary truth that this triangle is three-sided, since it couldn't possibly be a triangle without being three-sided; so three-sidedness is an essential property of this triangle.) 

Contingent Property: If it is only a contingent truth that an object O has a property P, then P is a contingent  property of O.
(Ex: While a triangle may have a side greater than three feet long, this is only a contingent fact, since it could possibly be otherwise -- it would still be a triangle even if it didn't have any side that big; so having a side greater than three feet long is only a contingent property of the triangle.)

Essence: the sum-total of an object's essential properties forms the essence of that object.