Jargon is considered the highly specialized vocabulary or language that is used in a specific profession. Jargon could be the words a mechanic uses to describe what went wrong with your engine: "Yeah, where the air-fuel mixture enters the crankcase, right at that reed valve there, is blocked, which means the crankcase stays pressurized."
Jargon could be the way doctors discuss a concept: "Epidural lysis of adhesions."
Jargon could be the language lawyers use to write, interpret, and manipulate the law: "The ratification of the conventions shall be sufficient for the establishment of the aforementioned limited liability corporation, pending a unanimous decision amongst involved parties."
In its broadest definition, the term "jargon" encompasses overly elevated language that serves no real purpose in the writing. It is embellishment, or added decoration. Although it sounds fancy, it isn't as effective at informing the reader as plain English is.
For example: I want you to assiduously, judiciously, but not self-floggingly evaluate your work.
When should you use jargon? Jargon is best used when you know your audience will be familiar with the terms--their definitions and uses--but even then jargon should be used only sparingly. Don't overwhelm your audience with overly specific jargon. When it is necessary to use jargon, let each term have its space in the writing. Let each term's complex meaning shine through and make meaning. Pile too many on top of each other and they'll lose their luster.
Here are some examples of jargon (the inflated English type) and their plain english counterparts:
optimal---best, most favorable
utilize---use, make use of
The next time you're at the doctor's office or have a repair man to the house, listen close for the specific language they'll use to discuss the situation. Listen for the jargon.