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.
There are a number of different ways in which you can use a semicolon in writing.
The most common use of the semicolon is to put together two independent clauses in one sentence. That is, you can use a semicolon instead of a conjunction to join two separate sentences. You could say for example, "She asked him a question; he answered it." There, you use a semicolon instead of saying "She asked him a question and he answered it."
Other than that, you typically use semicolons in various ways when you are making lists. Please follow the link for more details.
A semicolon is used to connect two similar thoughts into one sentence.