| Traditionally, relative clauses have been divided into two categories: restrictive (e.g. 'Teachers don't like students who don't work hard in class.' (1)) and non-restrictive (e.g. 'Teachers don't like students, who don't work hard in class.' (2)).
However, relative clauses may show logical relations between them and main clauses. For example, the relative clause in (1) specifies what type of students teachers do not like, and the sentence can be roughly paraphrased as 'Teachers won't like students, if they don't work hard in class.' On the other hand, the relative clause in (2) explains the reason why teachers don't like students, and the funny joke becomes very close in meaning to 'Teachers don't like students, because they don't work hard in class.'
The relative clauses in other sentences (e.g. 'The woman, who needs constant attention, is cared for by relatives.') and especially 'sentential relative clauses' (e.g. 'One of the boys kept laughing, which annoyed Jane intensely.') imply, depending on contexts involved, logical relations like 'cause and effect' between the relative clause and the main clause.
Relative pronouns establish a looser semantic connection between main clauses and relative clauses than conjunctions whose major purpose is to join parts of a sentence logically. Speakers/writers may intentionally or unintentionally use relative clauses when they wish to make a logically less explicit tactful sentence.
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