Data (computerized information) moves efficiently across the Internet by being broken up into little bits known as packets. Each packet is independently addressed and travels separately, and different packets can travel by very different routes. Imagine yourself wanting to send a really heavy textbook to a friend in another country. Instead of sending the entire book, you tear it into separate pages, put each one in its own envelope with a separate stamp and address, and mail all those envelopes one after another. Your friend may receive them at slightly different times, in the wrong order, but she can easily reassemble them into the book. Why would you mail a book in such an odd way? It turns out the Internet works best this way with everything broken into small, similar-sized chunks. (We have a whole article about how the Internet works that explains all this in more detail.)
When you download a file in the traditional way, you're effectively asking another computer (a server that sends out files to many different people) to send you zillions of packets one after another and you have to wait for all of them to arrive before you can do anything with any of them. With streaming, you start to use the packets as soon as enough of them have arrived. That's the essential difference. You can think of streaming as playing during downloading, but in fact the two things are different in all sorts of ways:
• Speed
Downloading: Unpredictable: Download time is unrelated to playing time. A music album could download in 5 seconds, 5 minutes, or 5 hours depending on its size, your net connection, and web traffic.
Streaming: Real time: Generally, a 1 hour video will stream in roughly 1 hour (though there may be occasional delays caused by buffering); unlike downloading, streaming media can be used for "live-live" transmission of events as they happen (also known as webcasting).
• Quality
Downloading: Uses traditional Internet packet communication (technically known as TCP/IP) with a system that automatically corrects errors. Any lost or damaged "packets" (downloaded chunks of data) are retransmitted. The file you eventually receive on your computer is an exact copy of the file that was on the server.
Streaming: Packet losses are ignored (lost and damaged packets are not resent), but that doesn't usually matter because digitally streamed video and audio is converted back into analog format before we watch it or listen to it. Any packets lost during streaming simply add "surface noise" to an audio stream or degrade the picture quality of a video (for example, with excessive pixelation (where the picture disappears into square blocks).
• File type
1. Downloading: A download is a single file with all the relevant data packaged together. So if you're downloading a movie, everything is packaged into a single movie file with a filetype something like MPEG4.
2. Streaming: If you stream a movie, each different part of the movie (sound, video, subtitles, or whatever) is transmitted as a separate stream. The movie player reassembles and synchronizes the streams as they arrive at your computer. In terms of bandwidth, these multiple streams are additive: if you're back in the Dark Ages with a slow 56 Kbps dial-up modem, you could happily watch a 20 Kbps audio stream and a 30 Kbps video stream together, but any extra streams would cause periodic pauses and buffering.
Servers
Downloading: Downloads work through traditional web-serving methods (technically known as the HTTP and FTP protocols) with any conventional web server. The same version of each file is served to everyone.
Streaming: Streams use RTSP (real-time streaming protocol) and need to run on a server specially geared for streaming. When you go to a web page that offers streaming media, you're generally redirected to a separate streaming server. There are typically different versions of each file that have been optimized for different connection speeds (for example, a low-quality version for dial-up and a high-quality version for broadband); in practice, different files are served to different people.
Encoding/decoding
Downloading: Files can be instantly uploaded to a server for immediate downloading.
Streaming: Files have to be compressed (perhaps using smaller video frames or fewer frames per second) and then encoded (turned into discrete, digital packets) before they can be streamed. People watching or listening to streamed files have to have appropriate decoding files installed on their computers (known as codecs) for turning encoded, computerized, digital files back into analog sounds and pictures that human ears and eyes can process. In practice, that means you need a plugin in your web-browser to handle whatever streaming media files you want to receive (and you'll need separate plugins for QuickTime, RealPlayer, and so on).
Multiple users
Downloading: The more people ("clients") download a file at the same time, the harder the server has to work, the slower it works for each client, and the longer it takes you to download—irrespective of how fast an Internet connection you have. (BitTorrent offers one solution to this problem.)
Streaming: In traditional streaming (unicasting), each client takes a separate stream from the server—necessarily, because different people will start streaming the same video or audio program at different times. Multicasting is a more efficient kind of streaming that allows a streaming server to produce a single stream that many people can watch or listen to simultaneously—for example, if lots of people are watching a football game live online at the same time. Some media players automatically use multicasting when they can.
Standards
Downloading: Downloaded files tend to be in standard formats (such as MP3) that play easily on any computer or operating system.
Streaming: There are three rival, proprietary streaming systems (more formally known as architectures—RealPlayer, Apple QuickTime, and Microsoft Windows Media Player), and though they're much more compatible than they used to be, it's not always possible to play files designed for one player on the others.
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