Some data is really streaming through your computer when you have a generative process that transmits data, which you can process on the fly or just discard, but not recall afterward unless you have stored it away in some data archival repository somewhere. It is like dragging water from a flowing river—the river keeps on flowing but you can filter and process all the water as it goes. It's a completely different strategy from processing all the data at once, which is more like putting all the water in a dam (an analogy for working with all the data in-memory).
As an example of streaming, we could quote the data flow produced instant by instant by a sensor or, even more simply, a Twitter streamline of tweets. Generally, the main sources of data streams are as follows: