The semicolon is certainly an interesting piece of punctuation. It isn't a comma—in fact, a comma would cause a run-on sentence. It isn't end punctuation—a semicolon must always be followed by an independent clause, and there always has to be end punctuation at the end of that clause for both clauses—before and after the semicolon—to be complete and correctly punctuated sentences.
The semicolon in type comes to us from a company in Venice, Italy and can actually be traced back to a particular printer and a passion. Aldus Manutius the elder (1449-1515) was a Venetian printer when it was becoming “in vogue” to preserve and publish Greek text by committing them to type (rather than hand-copied documents). Several Italian presses were making a name of themselves by publishing Greek classics, and Aldus concluded that Venice was the best place to begin his venture. Therefore, he worked to establish a printing house, the Aldine Press, after moving to Venice. He is credited with making books less bulky and more accessible to the common man, as well as providing better updated books through revised editions. Not only that, but he is also credited with the invention of the semicolon. He used semicolons in his typeset as a marker between antonyms and also as a way to separate “interdependent” clauses—two dependent clauses with some meaningful relationship that created a dependence of meaning while remaining grammatically independent. You can see examples of his use of semicolons in the image of an early Aldine-published text below.
Aldine Press gained a wide repute and Aldus contributed greatly to the field of type, pioneering work at his time. It is said that Gutenburg may have invented the printing press, but Aldine Press used it to revolutionize the world.
As mysterious as the history of the semicolon is for most people so is its correct use.
It is actually very simple. A independent clause is a complete thought—stick a period or question mark or exclamation mark at the end of it, and it is a sentence.
Examples of independent clauses:
I threw the ball.
A semicolons separates two independent clauses that are related to one another.
I'm glad I studied all week in preparation for the test; it was as difficult as I had expected.
I had not touched the flowers; I had not been in the office.
You could break this sentence up into two. It would not be wrong to separate them with a period, but the semicolon usage is more informative—it lets the reader know the sentences are connected without having read them—the semicolon tells them beforehand, “This independent clause's meaningfulness is linked to the independent clause before it.” The semicolon allows subtle nuances in meaning and connections. For example, in the first sentence, the semicolon could be replaced with the word “because” and maintain its same meaning!