You will usually use the semicolon to link independent clauses not joined by a co-ordinating conjunction. Semicolons should join only those independent clauses that are closely related in meaning.

Abdominal exercises help prevent back pain; proper posture is also important.

The auditors made six recommendations; however, only one has been adopted so far.

Do not use a semicolon to link a dependent clause or a phrase to an independent clause.

[WRONG] Although gaining and maintaining a high level of physical fitness takes a good deal of time; the effort pays off in the long run.

[RIGHT] Although gaining and maintaining a high level of physical fitness takes a good deal of time, the effort pays off in the long run.

Generally, you should not place a semicolon before a co-ordinating conjunction that links two independent clauses. The only exception to this guideline is if the two independent clauses are very long and already contain a number of commas.

[WRONG] The economy has been sluggish for four years now; but some signs of improvement are finally beginning to show.

[RIGHT] The economy has been sluggish for four years now, but some signs of improvement are finally beginning to show.

It may be useful to remember that, for the most part, you should use a semicolon only where you could also use a period.

There is one exception to this guideline. When punctuating a list or series of elements in which one or more of the elements contains an internal comma, you should use semicolons instead of commas to separate the elements from one another:

Henry's mother believes three things: that every situation, no matter how grim, will be happily resolved; that no one knows more about human nature than she; and that Henry, who is thirty-five years old, will never be able to do his own laundry.