The main task of the semicolon is to mark a break that is stronger than a comma but not as final as a full stop. It’s used between two main clauses that balance each other and are too closely linked to be made into separate sentences, as in these two examples:
The road runs through a beautiful wooded valley; the railway line follows it.
An art director searched North Africa; I went to the Canary Islands.
You can also use a semicolon as a stronger division in a sentence that already contains commas:
The study showed the following: 76% of surveyed firms monitor employee Web-surfing activities, with 65% blocking access to unauthorized Internet locations; over one-third of the firms monitor employee computer keystrokes; half reported storing and reviewing employee emails; 57% monitor employee telephone behaviour, including the inappropriate use of voicemail.