iTunes Custom RSS Tags - Keywords
In theory the iTunes Keywords tag improves search relevance when people search by topic rather than searching for the name of your podcast. So far, I haven't found that search functions in iTunes check for anything other than Title, Author and Description fields because the keywords I've injected don't deliver any results in a search. I'm hopeful this is a temporary oversight, because using keywords certainly lends itself to making the feed more meaningful considering we don't have mechanisms for searching inside the audio contents at the moment. Use this field to key on specific topics covered in your podcast to aid in search relevance. This tag is used for individual items in your podcast feed.
Based on these criteria, the Keywords tag will look like this:
<itunes:keywords>Keyword1 Keyword2 Keyword3 Keyword4</itunes:keywords>