Many years ago, a "media centre" was a PC that sat in your lounge room and served up video and music files. Luckily, you no longer need to pay for a bulky, noisy and expensive computer to watch your media on your TV.
In fact, even the dedicated media-streaming device is slowly on the wane, with many of its network functions slowly being incorporated into Blu-ray players, gaming consoles and even the TV itself, thanks to the ever-improving world of smart television.
Conversely, media streamers are edging into smart TV territory, with — for example — the WD TV Live Hub having apps for accessing Spotify, YouTube and even ABC iView.
A dedicated streamer will sometimes have a built-in drive (though that will add to the cost), but will nearly always have USB ports and the ability to play files directly from the ports, as well as network access, be it wired or wireless. The big advantage is that media streamers tend to have a broader range of support for file formats such as MKV, VOB, FLV, FLAC and others. They also tend to have good support for files with subtitles.
The joy of networked devices, however, is the chance for manufacturers to supply regular firmware updates, and the list of functionality on any of these devices stands a good chance of regularly growing. FetchTV, for example, is listed below, because a recent update gave it network-streaming capabilities for the first time ever.
Whether you do use a dedicated streamer or one of the other devices really depends on your home set-up, but, either way, a media streamer should be a core component of any home-theatre set-up.
(Note that, of course, the Sonos, Marantz and Pure don't support video, as they are audio only, but are perfect if high-quality music streaming is your main concern.)