The difference between tags and categories

We used to have just categories.
These allowed the broad grouping of post topics but when you wanted to describe a post in more specific terms you had to use more categories. That lead to very long category lists inside the blog, very long lists in sidebars. It was also confusing in that what we said were categories others said were tags.

So now we have tags as well.

Tags are similar, but instead of a list of words to choose from, you write them in a list (separated by commas) in a text box below your post. They are free form words and generally describe your post in more detail.

For example, a post titled “Last Night At Burger King” might be filed under the “Dinner” category, but could have tags like, “burger, chicken fries, chocolate shake, dr. pepper”.

Here’s the official announcement of our tagging feature.

Here are some common questions and answers:

Is Tag the same as tag ?
Yes. Capitals letters do not change a tag.

Is there a way to manage tags?
Can I see a list of all the tags I have used?
No, there is no way to view the tags you have used.

Is there a limit to the number of tags I can have?
No.

Urls.
A post into a category ‘food’, the url looks like this:
http://faq.wordpress.com/category/food/

The same post using the tag ‘food’ will look like this:
http://faq.wordpress.com/tag/food/

A post into the category ‘food’ and with the tag ‘rice’ will look like this:
http://faq.wordpress.com/category/food/
http://faq.wordpress.com/tag/rice/

Is there any advantage to using tags or categories or both?
Not here at WordPress.com and not for any search engines, no.

Do I have to use tags?
It is entirely optional.

Will categories be removed?
They are here to stay.

Have any links to any posts anywhere on my blog been changed by this?
They are all okay. We are intelligently redirecting your old URLs.

How do I delete a tag?
How do I rename a tag?
- You would need to find each post you had made with the tag and make the changes.

Are categories and tags hierarchical?
Categories can be treated that way by making categories children of another. This has always been the case.
Tags exist in their own right and have no set relationship to anything else.

I had a lot of categories. Can I delete those and just use tags from now? What will happen to my existing posts?
You can delete them but all your posts will go into the default category. You will have to edit every post and add back the tags.

The best place to find out more about what others feel about tags and categories is in the forums.