Playlists and smart playlists can be both exported from and imported into iTunes. There are mainly three reasons why you'd want to export a playlist: first, you could export the playlist to import it into another iTunes installation (like your laptop). Second, you can export a playlist to put it online, or email it to a friend, for example.
Exporting playlists only backs up the information related to the playlist; exporting playlists doesn't backup your music. This section about iTunes export formats is slightly technical, you can skip to backing up iTunes playlists.
The playlist is exported in "XML" format, which allows to display information in a simple way, which can then be used by any application, even outside iTunes. Here's a peak at the XML of an exported playlist:
This excerpt displays the most meaningful "fields" of information related to iTunes tracks; you will recognize them from the "Get Info" dialog that iTunes opens when right-clicking / control-clicking on a track.
You can also export your playlists in plain-text format ("Tab separated"), which can then be used, for example, by spreadsheet applications like Excel (the data can be imported or directly pasted in Excel).
In doubt, export your playlists as XML: the data is easier to manipulate, and can be directly uploaded on a website and blog, and fancily formatted using an "XSL stylesheet" to display a user-friendly format. (Perhaps more on that later.)
Since iTunes allows you to organize your music, take the time to backup your playlists and smart playlists. Having once faced a corrupted library after an iTunes upgrade, I now religiously backup my library and playlists, having had to spend hours re-creating them, re-assigning ratings to songs, etc. Backing up only takes a couple minutes, well worth your time.
Backup your exported playlists in a safe location: since the exported playlist file is plain text, it takes very little space. A playlist XML file can feasibly be uploaded or emailed to your self for safe keeping. As a rule of thumb, you should always have at least one backup at a remote location: either a geographically different, like a relative's house or your office, or virtually different, like an email or FTP server. This way, if your house burns down, you will still have your exported playlists to console yourself (provided you also backed up your music collection... This will be covered in another tutorial.)