Conjunctions are also known as joiners. They are used to link other words, clauses, or phrases together. Different kinds of conjunctions join different kinds of grammatical structures. Conjunctions can be used to create complex, elegant sentences to express complex ideas. Without conjunctions, all your sentences will be a series of short sentences written in the simplest forms.
There are three types of conjunctions: coordinating, subordinating, and correlative.

1. Coordinating conjunctions
They join equals to one another. They join words to words, phrases to phrases, and clauses to clauses. A coordinating conjunction goes in between the items joined. The seven conjunction words can be easily remembered by the acronym FANBOYS: For And Nor But Or Yet So

• For - Explains reason or purpose (just like "because")
• And - Adds one thing to another
• Nor - Used to present an alternative negative idea to an already stated negative idea
• But - Shows contrast
• Or - Presents an alternative or a choice
• Yet - Introduces a contrasting idea that follows the preceding idea logically
• So - Indicates effect, result or consequence

Examples:
The old castle seemed grand yet mysterious. (word to word)
What you say and what you do are two different things. (clause to clause)
I don't go for the fresh air nor for the ducks; I just like soccer. (phrase to phrase)
When a coordinating conjunction joins independent clauses, a comma is placed before the conjunction:
I want to work as a teacher in the future, so I am studying French at university.

2. Correlative Conjunctions
They come in pairs, and you have to use both of them in different places in a sentence to make them work. These conjunctions work together (co-) to connect two equal grammatical terms. So, if a noun follows a conjunction, another noun should follow the other pair of the conjunction.
Some examples of correlative conjunctions are:

• both/and
• whether/or
• either/or
• neither/nor
• not/but
• not only/but also

I will study both English literature and art history.
Clara not only wants money but also wants fame.
These pairs of conjunctions require equal (parallel) structures after each one.
When "and" is used with the last word of a list, a comma is optional:
He ordered pizza, fries, and coke.
He ordered pizza, fries and coke.

3. Subordinating Conjunctions
Subordinating conjunctions join two clauses together, making one clause dependent (or "subordinate") upon the other. A subordinate conjunction ties a dependent clause to an independent clause.
These are some common examples of subordinate conjunctions:
although as because
before how if
once since than
that though until
when whenever where
whether while why

The subordinating conjunction can go in the middle of the sentence. It is linked to the dependent clause, which can come before the independent clause.
You will never know until you try.
If the dependent clause comes first, use a comma before the independent clause.
Until you try, you will never know.

Conjunctions are important to create complex and compound sentences and help introduce complex ideas to your reader. If you want to know more about the different types of sentences, click here.