A sentence in which special emphasis is given to one part (e.g. the subject or the object) by using a structure with it or what. The purpose of a cleft is to put the clefted element into focus. For example, starting with the sentence John wants a mobile phone, we can form an IT-cleft to focus either on John or on a mobile phone.
It is a mobile phone that John wants. (Focus on a mobile phone)
It is John who wants a mobile phone. (Focus on John)

We can also create a WH-Cleft to focus on a mobile phone.
What John wants is a mobile phone or A mobile phone is what John wants.

Comparative

The term comparative refers to that form of an adjective or an adverb which is constructed either with –er or with more and which serves to express a higher degree of the quality denoted by the base word.

Examples are: bigger from big, worse from bad, more beautiful from beautiful and more carefully from carefully.
She is taller than her husband.
Russian is more difficult than Spanish.

English also has a comparative of inferiority, constructed with less, as in less interesting.

Complex sentence

A sentence which contains one main clause and at least one subordinate clause.
She said (main clause) that she would come (subordinate clause).
They rested (main clause) when evening came (subordinate clause).
A guest is unwelcome (main clause) when he stays too long (subordinate clause).
Tell me the news (main clause) as you have heard (subordinate clause).