Grammar: The Semicolon [ ; ]

 

The semicolon represents a break midway between a comma and a full stop, and is principally used to separate independent (sentence-like)
clauses that are strongly related 

      Never use a lathe or any other machine in the workshop if you are feeling
      ill or tired; it can be very dangerous.

A comma after tired would create a fused sentence. A full stop, on the other hand, might be used to create two sentences, but the logical
connection between the clauses (the second providing a reason for the first) makes a semicolon more appropriate.

We can often think of the semicolon as announcing the answer to the unspoken questions Why? or Why not? The semicolon then functions
as because 

      A wet election day is always good news for the Conservatives; their
      supporters turn out whatever the weather.

      (A wet election day is always good news for the Conservatives. Why? Because
      their supporters turn out whatever the weather.
)

      It is advisable to load a good dictionary on to your hard drive; the standard
      spellcheckers are not always reliable.

      (It is advisable to load a good dictionary on to your hard drive. Why? Because the standard
      spellcheckers are not always reliable.
)

      Machiavelli's prince does not obey the rules; he makes them.

      (Machiavelli's prince does not obey the rules. Why not? Because he makes them.)

In the same way, the semicolon can be understood to mean whereas, while and for 

      One of the principal advantages of being a sole trader is having no boss;
      one of the principal disadvantages is having unlimited liability.

      (One of the principal advantages of being a sole trader is having no boss, while (or
      whereas) one of the principal disadvantages is having unlimited liability.)

      There was nothing more the doctors could do for him; only time would tell
      whether he would recover.

      (There was nothing more the doctors could do for him, for only time would tell whether he
      would recover.
)

Finally, the semicolon can function as a listing comma when the items in the series are lengthy or include commas in other capacities 

      Many things need to be considered before starting your own business, such
      as a bank loan, which will normally require security; the location of the
      premises and whether to rent or purchase them; demand for the product or
      service; health and safety, especially if you are employing more than five
      people; and what prices to charge.

(See also lists.)