Clauses are the building blocks of sentences; without a good understanding of clauses you will not be able to write complex sentences.
A sentence is a complete group of words that come between two full-stops, every sentence has a subject and a verb.
A clause is not the same as a sentence, a clause is a group of words with a subject (s) and verb (v), however, one sentence can contain many clauses.
For example:
(s) Technology (v) is useful.
= 1 sentence with 1 clause.
Technology is useful, but it can be addictive.
= 1 sentence with 2 clauses.
Technology is useful, but it can be addictive, thus people must be cautious.
= 1 sentence with 3 clauses.
Technology is useful, but it can be addictive, thus people must be cautious in case they overuse it.
= 1 sentence with 4 clauses.
A phrase is a group of words without a subject or a verb. Be careful as, sometimes a phrase can be misidentified as a clause.
For example:
All over the world, people are celebrating the world cup.
= A sentence with a phrase and 1 clause.