CLAUSE

A clause is defined as a group of words that contains a finite verb.

Example:

I know that she is lazy.

In this sentence - know is a finite verb.

Thus 'I know' is a Clause. 

Also, 'she is lazy' contains the finite verb 'is'.

So she is lazy is also a clause.

Thus, we see that a sentence can contain more than one clause.

To simplify, we can remember that the number of clauses in a sentence will be equal to the number of finite verbs in that sentence. 

Every sentence has two types of clauses-

One is Main Clause/Principal Clause/Independent Clause

and the other is the Subordinate Clause /Dependent Clause