While historically the use of the semicolon and colon were similar, modern usage seems to be diverging, with semicolons indicating breaks slightly stronger than commas and colons introducing lists and explanations. \nSemicolons have always been used in a manner very similar to commas, but with greater "weight."  That is, the semicolon introduces greater breaks in the sentence, dividing clauses which are longer and more independent, while commas are reserved for short pauses and for separating lists of short items.\nContinuing up the continuum, colons are "weightier" than semicolons, but less so than periods.  They introduce a break which is longer or more important than that introduced by semicolons, but not as final as periods.  Colons separate clauses which can stand as independent sentences grammatically, while semicolons can also be used for non-sentential clauses.\nHowever, in modern writing the colon is rarely used in this role as glorified semicolon.  The colon has developed a specific semantics as a break separating an introduction from an explanation.  For example, the colon is used after the salutation in formal letters.  It is used to introduce quotes, separating the source from the quoted text.  A colon is used to introduce a list, included bulleted lists.  A colon is used to separate the parts in a multipart title, such as "Reading Lolita in Tehran: A Memoir in Books" or "Good to Great: Why Some Companies Make the Leap...And Others Don't".
When do you use a semicolon instead of a colon?