In the first generation of exchange model, advertisers were able to buy impressions in buckets of 1,000 through an auction-based – static bidding system.
Static bidding means that you are paying a flat or fixed average rate for each of the impressions in that bucket.
Real Time Bidding is dynamic bidding at the impression level. This means that an ad buyer, through the platform of their choice, can differentially value each individual opportunity to buy an ad impression in real time.
Here is the slideshare presentation of RTB in 30 seconds. Just hit the Play button and don`t forget to turn on your sound ☺