A conjunction is a word which merely joins two words, clauses or sentences. Note that a conjunction does not modify or qualify the words/clauses/sentences it joins.

Edwin and Melvin are brothers. (Here the conjunction and merely joins the two nouns Edwin and Melvin.)
She is beautiful but she is not intelligent. (Here the conjunction but merely joins the clauses ‘She is beautiful’ and ‘She is not intelligent’.)
You must work hard or you will fail. (Here the conjunction or joins the clauses ‘you must word hard’ and ‘you will fail’.)

Difference between conjunctions and relative pronouns

Both conjunctions and relative pronouns can be used to join two clauses. There are, nevertheless, some differences.

A relative pronoun is basically a pronoun. It is used instead of a noun. It also joins two clauses:

Read the sentences given below:

This is the book that she gave me.

Here the relative pronoun that is used instead of the noun book. It also joins the two clauses into one. This will become clear when you split the sentence into two.

This is the book. She gave me the book. (= This is the book that she gave me.)

Similarly,

This is the boy who got the first prize.

Here the relative pronoun who stands for the noun boy. It also joins the two clauses into one.

This is the boy. The boy got the first prize. (= This is the boy who got the first prize.)

A conjunction merely joins two clauses. It doesn’t qualify any other word in the sentence.

Man is known for his valor and woman is known for her vanity. (!?)

Here the conjunction and merely joins the two clauses into a large sentence.

Difference between a conjunction and a preposition

A preposition not only joins two words but also modifies a noun/pronoun and tells about its relationship with another word or words in the sentence.

This pond abounds in fish.

Here the preposition in not only joins the words abounds and fish, but also shows the relationship between the noun fish and the rest of the sentence.

My house is situated near the post office.

Here the preposition near not only joins the words situated and post office, but also shows the relationship between the noun post office and the rest of the sentence.

However, as explained in the first paragraph, conjunctions only join two words, clauses or sentences without modifying or qualifying any of them.