These two forms of punctuation are often used incorrectly for one another.
Colons can be used to direct a reader to examples or significant words:
His main flaw is his downfall: egotism.
Also, they direct a reader to a list of things:
A lot of vegetables are the same color: lettuce, peppers, snow peas, and celery.
Where a colon can be confused for semicolon usage is when it joins two sentences and the second sentence illustrates the first:
Most critics agree on one point: Titanic was his crowning achievement.
Semicolons are mainly used to combine two disjointed, yet related, thoughts.
As I drove home I reflected on the movie; it had a certain quality to it that made me reminiscent.
Also, semicolons mark more emphasis than just a comma and can be used like so:
Hand Sally the tomato; it belongs to her.
Semicolons can be used to separate one list from another:
We learned the basics of grammar, punctuation, and spelling; the ins and outs of brainstorming and getting started; and how to take notes in an interview.
Semicolons can also join independent clauses that are joined by words like however, therefore, nevertheless, moreover, etc…
He started his career in the theater; however, he quickly made his way into films.
Semicolons are not used to introduce quotations or lists.
Colons are not interchangeable with semicolons either.