It is always better to write plainly and be understood than to have style and be misunderstood.
Use a comma when you want to take a breath, mentally or physically.
Use a colon when you want to make an example of something: For example, just like this.
Use a semi-colon where, in conversation, you would raise your eyebrows to make a point.
Keep your sentences short and jargon-free.
Taken from a superb post by John Scalzi.