Here's a Wikipedia definition of streaming
So, your computer/TV/phone can either download the entire file (like email) -or- allow you to watch/listen to a "stream" of content. It doesn't save the file, you just enjoy it, as it "streams" by.
Here's an approximate list of how big the stream needs to be to carry the content you want. Although streaming music doesn't take much bandwidth (there are lots of free internet radio stations) - video does. High definition video even more.
Those of you who stream video, like Netflix, may notice that picture quality might vary. When the internet doesn't have enough bandwidth (stream size), it might provide a lower resolution picture. In the Summer, you probably notice it more because there are more people -sharing the stream.