An OTA – short for Online Travel Agency – is a travel agency that sells its goods and services such as flights, hotel stays or rental cars, (mainly) online. Very often these OTAs also offer so-called dynamic packing. That’s when two or more individual services are combined to one package to save costs. One example would be a packaged deal that includes a hotel as well as a rental car at your vacation spot.
Especially when booking flights, OTAs are an excellent way to reduce the price. They will often sell tickets cheaper than the airlines themselves! In some cases they even put up with financial loss, just to acquire customers. Most OTAs earn money by selling travel insurances.
Another term that you could come along sooner or later is the OTA Glitch. What it means is an error in one of the OTAs systems resulting in very low prices, e.g., for a specific flight.
Two of the big players are: