There are two major types of clauses main (or independent) clause and subordinate (or dependant) clause.

Main Clause and Subordinate Clause – Comparison

He is buying a shirt which looks very nice.

The above sentence has two clauses “He is buying a shirt” and “which looks very nice”. The clause “He is buying a shirt” expresses a complete thought and can alone stand as a sentence. Such a clause is called main or independent clause.

While the clause “which looks very nice” does not express a complete thought and can’t stand as a sentence. It depends on another clause (main clause) to express complete idea. Such a clause is called subordinate or dependent clause.

Main or Independent Clause
“Main (or independent) clause is a clause that expresses a complete thought and can stand as a sentence.

Examples
I met the boy who had helped me.
She is wearing a shirt which looks nice.
The teacher asked a question but no one answered.
He takes medicine because he suffers from fever.
He became angry and smashed the vase into peaces.
In the above sentences each underlined part shows main clause. It expresses complete though and can stand as a sentence that is why a main or an independent clause is normally referred as a simple sentence.

Subordinate or dependent Clause
Subordinate (or independent) clause is a clause which does not express complete thought and depends on another clause (main clause) to express complete thought. Subordinate clause does not express complete idea and can’t stand as a sentence. A sentence having a subordinate clause must have a main clause.
Example
He likes Chinese rice which tastes good.
The clause “which tastes good” in above sentence is a subordinate clause because it does not express complete thought and can’t stand as a sentence. It depends on main clause (he likes Chinese rise) to express complete thought.
Examples.
I met the boy who had helped me.
I bought a table that costs $ 100.
He takes medicine because he suffers from fever.
The teacher asked a question but no one answered.
Subordinate (or dependent) clauses are further divided into tree types,
1. Noun Phrase, 2. Adjective Phrase, 3. Adverb Phrase.