The semicolon (;) is a punctuation mark which indicates a pause, and can be seen as a super comma because it can be used correctly where the use of a comma would be incorrect. That does not mean that a semicolon can be used everywhere a comma is used.
A common mistake some people make is to join two sentences with a comma, but without a conjunction. Such sentences can, however, be joined by using a semicolon.
Mary is French. Jane is Chinese. - correct
Mary is French, Jane is Chinese. - incorrect
Mary is French, but Jane is Chinese. - correct
Mary is French; Jane is Chinese. - correct
They are used to connect closely related ideas, and when used correctly, can get your writing a better mark in the exam.
I never use a computer; I prefer pen and paper.
Semicolons are used for lists that contain commas, to avoid confusion, and for lists where the elements are groups of words.
I bought some apples, pears, and bananas from the greengrocer; some sausages, pork chops, and bacon from the butcher; and some cakes from the cake shop.
But, although a comma is used in letter writing at the end of the opening greeting, it would be incorrect to replace that comma with a semicolon.
Dear David, - correct
Dear David; - incorrect