A conjunction is a word that joins words, phrases, clauses, or sentences.

e.g., but, and, because, although, yet, since, unless, or, nor, while, where, etc.

Examples.

  Conjunction Joining Words:

• He bought a book and a pen.   
• She ate an apple and a mango.
• I forgot to bring my laptop and camera.

  Conjunctions Joining Phrases:

• You can meet me at my house or at my office.
• He always sits in the classroom or in the library.  

  Conjunctions Joining Clauses:

• I waited for him, but he did not come.
• You cannot succeed unless you work sincerely.
• We saw a man in the street who was crying.
• I sent him an email, but he did not respond.
• She laughed and entered the room.

  Conjunction Joining Sentences:

• I went to a market where I saw a joker.
• I saw him while I was driving my car.
• He studies biology because he wants to become a doctor.

  A sentence can have more than one conjunction. See the following example.

• He bought a red shirt, but I bought white shirt because it was cheaper. (two conjunctions).
• I like physics because I want to become an engineer, but my friend likes biology because he wants to become a doctor. (three conjunctions). 

The conjunction can either be a single word or a group of words known as compound conjunction.

Single-word conjunctions: but, and, because, yet, since, unless, etc.

The usage single-word conjunctions can be seen in the above examples.

Compound conjunctions: as long as, as far as, as well as, in order to, even if, so that, etc.

The usage of compound conjunctions in sentences can be seen in the following examples.

Examples.

• You live a happy life as long as you think optimistically.
• I will not tell him the secret even if he insists a lot.
• He bought a laptop as well as a bag for it.
• Always speak the truth in order to avoid problems in life.
• He as well as I had gone to a cinema.
• As far as I know him, he cannot cheat someone.
• He is coming here so that you can meet him.

There Conjunctions are classified into three main types:

1. Coordinating Conjunctions

These connect words, phrases, or independent clauses of equal importance.
Examples: and, but, or, nor, for, so, yet

• I wanted to go for a walk, but it started raining.
• She bought apples and oranges.

2. Subordinating Conjunctions

These connect an independent clause with a dependent clause, showing cause, time, condition, or contrast.
Examples: because, although, since, unless, while, when, if

• I stayed home because I was feeling sick.
• Although he studied hard, he did not pass the exam.

3. Correlative Conjunctions

These are pairs of conjunctions used together to link balanced words, phrases, or clauses.
Examples: either…or, neither…nor, both…and, not only…but also

• Either you finish your homework or you cannot watch TV.
• She is not only intelligent but also very hardworking.

Click here to read more about the types of conjunctions.