The definite article the is used in the following cases:

1.    When a singular noun represents a whole class.

The camel is a beast of burden.
The spider has eight legs.
The computer is a useful machine.
The apple is red.
The rose smells sweet.

Notes:

Instead of a singular noun, it is possible to use a plural noun. But note that the article the is not used before a plural noun representing a whole class.

Camels are beasts of burden.
Spiders have eight legs.
Apples are red.
Roses smell sweet.

The is never used before the nouns man and woman when they represent the whole class.

Man is mortal. (BUT NOT The man is mortal.)

We use a/an and not the when a human being of any profession or occupation is intended to represent his/her class.

A teacher must have patience. (BUT NOT normally The teacher must have patience.)
A carpenter makes furniture.
A book-binder binds books.

2.    To talk about a person or thing which has already been referred to:

The boy who came to see me was my brother. (Which boy? The one who came to see me)
The story he told me was very interesting. (Which story? The one he told me)
He promised to take the medicine I gave him.
Call the boy waiting outside.
Notes:

Note that we use a/an, and not the, when a noun is introduced for the first time.

Compare:

I saw a girl in the library.

The girl I saw in the library was reading.

3.    While referring to scriptures or classical books:

The Iliad, The Mahabharata, The Ramayana, The Bible, The Quran, The Vedas etc.

Note that we do not use the article, when the book is referred to with the author’s name.

Shakespeare’s Hamlet (NOT Shakespeare’s the Hamlet)
Homer’s Iliad

4.    When referring to the names of newspapers and journals:

The Economic Times
The New York Times
The International Herald Tribune
The Times

5.    While referring to the names of mountains, peaks, hills, glaciers, rivers, canals, oceans, seas, archipelagos, straits, sea channels etc.

The Himalayas, The Alps, The Everest, The Nile, The Thames, The Panama Canal, The English Channel, The Indian Ocean, The Bay of Bengal, The Strait of Gibraltar, The British Isles