Difference between description and content:encoded tags in RSS2

The <description> tag is for the summary of the post, but in plain text only. No markup.

You can get around that if you escape the tags or wrap the content in:

<![CDATA[ post body goes <strong>here</strong> ]>

But you're not really supposed to be doing that.

If you want markup, you're supposed to use <content:encoded> and use the <![CDATA[ and ]> wrappers here.

The content:encoded element can be used in conjunction with the description element to provide an item's full content along with a shorter summary. Under this approach, the complete text of the item is presented in content:encoded and the summary in description.

RSS Best Practices Profile: content:encoded

In short, <description> is for the summary and the rest of the post is in <content:encoded>.

Since some readers may not support the tags as expected, you usually see the entire post in <description> and not a mix of the two.

The previous answer is incorrect.

The RSS 2.0 spec is very clear that you can encode the value of the <description> element.

An item may also be complete in itself, if so, the description contains the text (entity-encoded HTML is allowed; see examples), and the link and title may be omitted. All elements of an item are optional, however at least one of title or description must be present.