A colon is a punctuation mark and, much like the comma, it works to organize sentences and break everything down for coherency purposes.
Colons are most commonly used in the following ways:
1. To bring attention to a specific point: the colon draws the eye and allows some breathing space in between each part of the sentence. It also often explains the first part of the sentence in greater detail. A very long sentence appears less daunting when a colon is used.
2. To indicate the beginning of a list. For example, Living Healthy in the Real World is updated three times a week on the following days: Mondays, Wednesdays, and Fridays.
Depending on where you live in the world, and which style guide you are following (such as MLA or APA), there will be different rules regarding capitalization after a colon. As a general rule, however, you can use lowercase letters following the colon if you are using it to emphasize a point. Think about it the way you would a comma or a semicolon; the second part of the sentence remains a part of the sentence, and thus should be lowercase letters. Regardless of which style guide you are following, always use capitals as normal for names of places etc.
Colons usually suggest a more formal manner. More often than not, for informal pieces, we choose words and sentence structures which do not make use of colons. The exception to this is when we are creating a list that is more easily read with a colon preceding it. Two instances in which you might use a colon in a formal document are after the salutation in a letter and before quoting someone.
A word of caution
Be wary about grammar! Colons might break up sentences, but that does not mean that we should forget about the importance of grammar. You can avoid this problem by using a structure of subject-verb-object in the first part of the sentence, rather than just using a subject-verb combination.
In the above example, the following is the object that polishes off the first part of the sentence nicely. Colons should also not be used after a proposition (such as the word “of”).
So why use it?
Although colons should not be used with great frequency in a regular piece of writing, they are highly useful when organizing lists and also when we want to make a dramatic statement. Colons make the reader stop and pay more attention to a sentence, so it is an excellent way to really emphasize your point. The less you use them, the more emphatic they will be when you create a sentence with one. Choose wisely!