The first recorded use of the semicolon in English is in 1591; its cause was then championed a few years later by The Alchemist author Ben Johnson. Since then it has fallen out of common usage, superseded largely by a dash, but the rules governing it are relatively simple, and once grasped it can be an effective tool in essay writing.
According to Encyclopaedia Britannica, the semicolon ranks mid-way between a comma and a full stop. In essence this means that its role is to link two closely-related yet independent clauses which do not benefit from a conjunction between them. Each clause must be grammatically complete in itself and the latter will not necessarily begin with an upper case letter.
So, a writing tutor might demonstrate the use of a semicolon thus: "I went to see the castle; I was told it was closed for restoration work."
Semicolons can also be used to join two clauses that have a conjunctive adverb between them, such as 'however', 'therefore' and 'otherwise'. For example: "I couldn't decide which dress I wanted to wear; therefore I packed them both."
The final major use of a semicolon is between items in lists where internal punctuation is used. This provides clarity, clearly demarcating each item. An example might be: "The artist had used many cities for inspiration: Marrakech, Morocco; London, England; Budapest, Hungary; and New York, United States of America."