A "datastream" is the service that sends data from one place to other places. The starting point is data that has been collected by AEP Edge, usually from Web SDK or Mobile SDK. The ending points can be one or more of Adobe's services.
You can think of it like a water pipe in your house. The water pipe connects from your neighbourhood's main water supply to your house's taps, toilets, shower, washing machine, and other appliances that need water. The same water is sent to the appliances, but each appliance uses the water differently, e.g. the water for the toilet is used for flushing waste, the water for the washing machine is used to clean your clothes, and so on. Later, if you have a new appliance, like a water heater, you can connect it to your water pipe too, so that it too can receive the incoming water.
Likewise, the datastream is like a water pipe. AEP Edge is the "water supply" sending streams of data to you. The endpoint services that you configure in your datastream, like Adobe Analytics, Adobe Target, Event Forwarding, etc. are the "sinks" and "toilets" and other "water appliances". All of these services receive the same "water" or data coming from your AEP Edge "water supply". But each of these services do different things with your data, e.g. data that goes to Adobe Analytics is used for reporting, data that goes to Adobe Target is used for A/B testing, and so on. You can update the connected services at any time, e.g. later on, you may connect Adobe Audience Manager to the datastream, and the data that goes to Adobe Audience Manager will be used for audience profiling.
Hope that helps!
