A semicolon is a tool of punctuation. You use it to join two independent clauses (complete sentences) together into a compound sentence.
The semicolon is a useful piece of punctuation when you want to combine two complete sentences together into a single sentence because they share a common idea or theme. If the topic of the sentences are not related, you should not use a semicolon. It is best to treat those as two separate sentences in order to keep your writing clear, succinct and logical.
INCORRECT: An ear of corn made a snotty remark while his friend burned to death next to him; autumn is the loveliest season of them all. (Here, the first and the second clause have nothing to do with one another, so it makes no sense to combine them into a single compound sentence.)
CORRECT: An ear of corn made a snotty remark while his friend burned to death next to him; he was a very inconsiderate ear of corn. (This makes more sense because they both describe something about the interaction between two ears of corn.)
A common mistake is to use a semi-colon with a coordinating conjunction (‘for,’ ‘and,’ ‘nor,’ ‘but,’ ‘or,’ ‘yet’ and ‘so’) in a compound sentence. If you want to use a coordinating conjunction, use a comma to separate the two clauses.
INCORRECT: One ear of corn was wise enough to wear sunscreen; but the other ear ended up popping on the beach. (Here, the coordinating conjunction ‘but’ indicates that a comma is appropriate instead of a semi-colon.)
CORRECT: One ear of corn was wise enough to wear sunscreen; however, the other ear ended up popping on the beach.