How to Generate Tags from Description (input)?

Looks like acts_as_taggable_on is what you need. The example included
on that page should make it pretty easy to do.

http://github.com/mbleigh/acts-as-taggable-on/tree/master

You're trying to pick out keywords out of the description to use as
tags? You could use a language parser to pick out sentence components
- Subject, Direct Object - and use those as tags.

http://deveiate.org/code/linkparser/

g.