I think the severity of it, and the things that trigger it, are different for everyone - but, basically, it's the body's "fight or flight" response being activated for no good reason. It's a bolt of adrenaline going through the system which can cause rapid heartbeat, shaking, sweating and a sense of heightened awareness to everything. I used to get them all the time though they've tapered off a bit of late. I had a mild one about a month ago but I just calmed myself before it got too far. If you're in public it can be more intense because you panic about being seen in a state of panic...and so it can intensify.