The convention was first proposed over 50 years ago and is now so common the latest international rules state it does not even need to be alerted, while an old fashioned penalty double does!
They occur when a) partner has opened & b) the next opponent overcalls. A double by you is now said to be "Negative". This means you do NOT hold partner's suit and you do NOT hold the opponent's suit. The inference is you therefore hold the other two suits. You will have - depending on the level of the overcall - about 7+ HCP. An extension to this idea is that if either partner or the overcaller has bid a Major suit, you will hold the OTHER major with at least 4 cards, and not necessarily the 4th suit as well. Here's an example of it in action: Partner opens 1C and your RHO bids 1S. You hold
You do not have enough to bid at the 2 level and 1NT would promise a decent Spade honour, so it looks like you lie or are frozen out of the auction. NOT SO! You simply Double. Now partner knows you have at least 4 H's and (in an ideal world) 4 D's as well. Armed with this valuable information partner will be able to make informed decisions later in the auction, rather than trying to feebly guess what's going on if you just pass, or lie.
The reason for using Double like this - as a positive forward going manoeuvre and not simply a weapon to bash opponents - is to do with numbers. Specifically, the numbers of hands possible in relation to the number of bids possible. There's in the region of 53 trillion possible Bridge deals and to bid them all we have a mere 35 legal suit bids. However we can vastly improve our chances if we use DOUBLE as a positive, forcing bid in the early rounds of bidding. Now we have 36 bids! Modern theory has also begun - in certain competitive situations - to use PASS as a forcing bid as well, so adding 2 whole bids to the armoury in our fiery battle against random chance.
Remember, Double is a bid just like any other, and like any other bid, it can have different meanings in different situations.