Simple meta tag rules (from google webmaster tools):

1. do not add useless meta tags
2. add site-wide-unique meta tags
3. add max 10 keywords
4. in doubt add no meta tags, search engines will grab something

Obey or be penalized.

For simplicity use: MetaOnRails Rails plugin

Usage

#in your head (no not that head...)
=display_meta

#in your views, add unique keywords/description that matter
set_meta(
  :keywords=> [@movie.category, @movie.genre,@user.name]*' ',
  :description=>@movie.description
)

Output

<meta name="description" content="my description" />
<meta name="keywords" content="my,keyword" />