Jack: Hey! Let's go decrease WorldSuck!
Harry: Okay! We can donate to Kiva dot org!
Colton: Whatever, you guys, let the third world countries take care of themselves. Sharon's hot, isn't she? I'd tap that.
Jack: Colton, you are such a Decepticon.
Harry: In my pants.
Most stories suggest they began as a rebel faction on Cybertron which invented arms for the first time and sought to control other transformers in order to dominate the universe, although I find it more likely that they were actually the dominant faction from the start which gradually became more and more arrogant as their power increased. (This is more consistent with the fact that the Decepticons are in control and the Autobots are the rebels in nearly all the early TransFormers stories set on Cybertron).
Hell-bent on interstellar domination, including on earth where they crash-land, the Decepticons attempt to seize and amass energy when and wherever they can, without any concern for other beings or for sustainability. Many Decepticons are also depicted as ruthless and malevolent entities in their own right, enjoying destruction and violence as a good in itself (although the attribution of such emotions to robots seems problematic).
The Decepticon symbol is a purple face insignia looking similar to Soundwave's head. In the cartoons, they also always fire purple/magenta laser beams.
Led by Megatron and later by his reincarnated form, Galvatron; also in various media by other characters such as Ratbat, Soundwave, Starscream, Scorponok, Bludgeon, Thunderwing and Lord Straxus.
George W. Bush is intent on a Decepticon-style oil grab in the Middle East.
That van is painted in a very scary way; it looks almost like it should be a Decepticon.
Often, a decepticon is used to "soften the blow" of bad news or to veil a threat.
Boss: "If you don't work harder, there might have to be another round of layoffs :)"
Employee: "Were you really smiling when you typed that, or was that smiley a decepticon?"