The semicolon, colon, and dash are three punctuation marks that are frequently confused with one another, though they serve three distinctly different purposes. Though they are not used as often as, say, the period or the comma, they are interesting to learn and use on occasion.

      A grammar teacher would probably explain the semicolon (;) by saying that it separates two main clauses that are not connected by a coordinating conjunction. In layman's terms, the semicolon is used in place of a period to conjoin two complete sentences that are similar or connected. In the following example, a period is used to end the first sentence and make two separate sentences.
           EX: The bartender turned away the three boys. They were all underage.

In this instance, semicolon use would be more effective after the first sentence because the phrases relate to each other. Here, the second sentence explains the actions in the first. Though both ways are technically correct, the semicolon is the better option.
         EX: The bartender turned away the three boys; they were all underage.

Note that after the semicolon, use one space and make the first letter of the second phrase lowercase. Semicolons can also be used between phrases that are joined by a conjunctive adverb (however, thus, otherwise, nevertheless, etc.).
          EX: I wanted to buy a black laptop; however, the store only had blue in stock.

Correct semicolon use can be tricky. Though some say it is an old-fashioned punctuation mark that is out of style, others will swear that it enhances writing. Either way, it's better to know how to use it; it may come in handy someday.

      Though their names are similar, the colon (:) serves a very different purpose than the semicolon. Colons are used mainly to set apart a list or series from a sentence. A main clause must be before the colon, but doesn't necessarily have to be after. Colon use after a sentence fragment is a very common mistake. Notice the fragment in the following sentence.
EX: I like to play video games when I am: bored, tired, and alone. (Wrong)
A better way to write that sentence would be to take out the colon entirely. Again, colon usage is correct when it follows a complete sentence.
EX: I went to the grocery store and bought three things: peanut butter, jelly,
           and bread.

Here, a main clause is followed by a colon with the list at the end.

      Lastly, the dash (–) has two main purposes. First, they are used to set off additional, but not essential, information within a sentence. This can be done with commas, but the dash offers more emphasis on the information than a comma. Parentheses are used to do this as well, but they provide less emphasis than commas, making the dash the most potent of the three. Here is a sentence with correct dash usage,
         EX: I asked my three best friends—Karly, Bob, and Amy—to come over and
          celebrate my birthday with me.

The dash is also used at the end of sentences to summarize to emphasize a point.
          EX: I opened my lunchbox and saw the one food I hate more than any other—

      Though these three punctuation marks are important to know and understand, use them sparingly in writing. This way, readers are not inundated with them and the ones used will stand out more.