Descriptions and keywords can be added to your posts and pages by using custom fields. For Descriptions you would use the custom field name: description and for keywords you would of course use the custom field name: keywords. Simple as that. If you are unfamiliar with custom fields you can check out this page in the codex for greater detail. Here are the basics of what you are looking to do.
After you have entered your post or page scroll down a few and you will see a post box titled “Custom Fields”. Under the heading Name, you will see a drop down and just below that is Enter New. Click Enter New and type your key, such as, description. In the Value field to the right you would enter your custom description.
The same steps can be followed with your keyword, be sure that they are comma separated. To answer the question ahead of time, these are not the same as nor will the keywords be generated from Post Tags.