Discharge rate
This is simply how fast a battery can be discharged safely. In RC LiPo battery world it is called the ’C’rating. A battery with a discharge rating of 10C means you could safely discharge it at a rate 10 times more than the capacity of the pack, a 15C pack = 15 times more, a 20C pack = 20 times more, and so on.
On a 20C rating on a 2000 mAh battery: 20 x 2000 = 40,000 milliamps or 40 amps. Time wise, a 40 amp draw on this pack would exhaust it in about 3 minutes (2000/60min = 33.3 per min. multiplied by 20c = 666 mAh per minute - divided into the packs capacity of 2000 mA = 3 minutes). Now that’s a lot of math!
Most RC LiPo Battery packs will show the continuous C rating and some are now indicating a burst rating as well. A burst rating indicates the battery discharge rate for short bursts of extended power. An example might be something like “Discharge rate = 20C Continuous/40C Bursts”
The higher the C rating, usually the more expensive the battery. This is where you can save some money. Getting an extremely high discharge rated pack when there is no way you could possibly pull the full amount of power is not required but it won't hurt either. The most important thing is you can't go with too low a discharge C rating or you will damage your battery and possibly your ESC (electronic speed control).
So how do you know what C rating to get when purchasing your LiPo RC Battery Pack? The easy answer most will give is to get the largest C rating you can. RC LiPo packs are coming down in price all the time. If you find a 35C pack for the same price as a 25C when that is all you need, go for the 35C pack - it will run cooler and have a longer life span. Like most things, pushing a Lipo pack hard close to its limits will wear it out and reduce it's useful capacity in very short order. If however you get a pack with a C discharge rating at least double of the maximum you intend to pull out of it, with proper care, there's no reason you shouldn't be able to get at least 400 charge and discharge cycles out of it with little degradation.
You could get a 10,000mah battery in just 10 grams and once these sorts of battery's go into production the possibilities will be endless for battery powered devices. You could run laptops for weeks if not months same as phones and our beloved RC flying machines can be flown for hours.
