He he ... okay, let's have another go ...
The normal search only uses the words people post, and can't really second-guess what you're searching for or find similar things.
For example, in a thread about trikes if no one posts the phrase "three-wheeler" then the regular search system won't find it. However, if someone 'tags' the thread with the "three-wheeler" phrase then you'll be able to find a thread about trike by searching the tag cloud for "three-wheeler". Does that make any sense?
Cheers,
Shaun