Most streams are of the first-in-first-out nature, thus some streams are also implemented as queues.
Streams may be representations of large amounts of data, and as such, these amounts of data may be processed little by little (block by block, line by line, etc.) and prevents such errors as data overflow.
Streams are often used, but not limited to, files, which may occassionaly contain large amounts of data.
Information may also be "streamed" between programs, storage, and even computers. For example, your media player may stream audio from your CD drive. It's not conventional to copy the whole music file into memory, as it may slow down your computer. It's possible to copy the file into your hard disk and play, but it's also not conventional, as you would have waited for a minute after you press play before it actually played.