Articles are either definite or indefinite.
The is the definite article, since it points out a particular individual, or group, or class.
An or a is the indefinite article, because it refers to any one of a group or class of things.
An and a are different forms of the same word, the older ān.
The most common use of the definite article is to refer to an object that the listener or reader is already acquainted with; as in the sentence,—
Don't you remember how, when the dragon was infesting the neighborhood of Babylon, the citizens used to walk dismally out of evenings, and look at the valleys round about strewed with the bones?—Thackeray.
NOTE.—This use is noticed when, on opening a story, a person is introduced by a, and afterwards referred to by the:—
By and by a giant came out of the dark north, and lay down on the ice near Audhumla.... The giant frowned when he saw the glitter of the golden hair.—Heroes Of Asgard.
The is often prefixed to the names of rivers; and when the word river is omitted, as "the Mississippi," "the Ohio," the article indicates clearly that a river, and not a state or other geographical division, is referred to.
No wonder I could face the Mississippi with so much courage supplied to me.—Thackeray.
The Dakota tribes, doubtless, then occupied the country southwest of the Missouri.—G. Bancroft.
The most frequent use of the indefinite article is to denote any one of a class or group of objects: consequently it belongs to singular words; as in the sentence,—
Near the churchyard gate stands a poor-box, fastened to a post by iron bands and secured by a padlock, with a sloping wooden roof to keep off the rain.—Longfellow
When the indefinite article precedes proper names, it alters them to class names. The qualities or attributes of the object are made prominent, and transferred to any one possessing them; as,—
The vulgar riot and debauchery, which scarcely disgraced an Alcibiades or a Cæsar, have been exchanged for the higher ideals of a Bayard or a Sydney.—Pearson