Categories and Tags

What categories and tags should I use for this blog? And what is the difference between categories and tags? I understand that categories are used for use within the blog and tags are used within the WordPress.com environment.

Therefore I’ll:

• Use the ‘postaday2011’ category and tag for all posts.
• Use a ‘draft-blog-post’ category so that I can easily find posts which I’m thinking of working up to a full blog post.
• Not use the ‘Uncategorised’ category for a post for longer than a week.

I’ll add any new policy decisions below, so that I’ve got a record.