For what is the semicolon [;] used?

There are several uses for semicolons that are called such
since they are composed of one-half of a colon and a comma.  A semicolon is stronger
than a comma, but not as forceful as a
colon.

1.  Use a semicolon between
independent clauses that are closely related in though and that are not joined by a
coordinating conjunction. (Using a semicolon in this way makes a stronger
statement since the second clause is equal to the first in emphasis without a
conjunction)

e.g. We left at sunrise; we stopped at
sunset.  [NOTE:  Be careful to use the semicolon in this case.  Using a comma is a
serious error in writing courses--called a comma
splice]

2.  Use a semicolon
between independent clauses joined by a conjunctive adverb or a transitional
expression.

e.g.  The snowfall made
traveling difficult; nevertheless, we arrived home safely.  (See the first site below
for more conjunctive adverbs)

e.g. Dennis plays baseball
well; in fact, he would like to try out for the college's team. (See the second site
below for more transitional words and
phrases)

3.  Use a semicolon between items in
a series if the items contain commas.

e.g. 
The club's pesident has appointed the following people to chair the standing
committees:  Thomas Payne, planning;  Rebecca Washington, membership;  Quincy Adams,
financial; and Anne Wright, legal.

4.  A
semicolon (rather than a comma) may be needed before a coordinating conjunction to join
independent clauses that contain
commas.

e.g.  I wanted to register for
biology, volleyball, and converstional Spanish; but only calculus, golf, and
intermediate French were available during late
registration.

Always remember that the purpose of any
punctuation is clarity.  So when commas will not separate ideas or lists sufficiently,
use the semicolon.