It is important that everyone adds relevant tags to their deals. They go in the "tags" field, and can be a single word, or multiple words. Tags are separated by commas (,). In fact, any member can add tags that you think are relevant to any deal, even if it isn't their own... You do this by pressing the "Edit" button on the deal.
Why should I tag deals?
Tags are a really useful way of creating groups of items. For example, the "DVD" tag will show you lots of DVDs. The "PS3" tag will show you lots of games for the PlayStation 3. The "Nokia" tag will show you lots of "Nokia" mobile phone handsets and, more specifically, the "Nokia 6300" tag will show you all Nokia 6300 mobile phone handsets, and maybe even threads in other sections (such as "Miscellaneous") which might give you some personal reviews on the item before buying it!
So, they're very, very useful.
Another thing that you can do with tags is create a custom RSS feed. You can do this in the Custom Settings window (you must be logged into the site). If you add "PS3" as a tag, then you'll see all deals that people tag with "PS3". This is a lot more specific than an RSS feed which just shows deals from the "Entertainment" category, for example.
Is that it?
There's a lot more scope than I've talked about here... For example, when posting DVDs as deals, I usually just add the title of the DVD, "dvd" and "dvds" as tags. The possibility is there to add leading actors/actresses in the film and even the director, allowing you to find other DVDs with a similar cast or director.