Joomla 1.6 has made it quite easy to add meta data to a category.
Adding Metadata to a Joomla 1.6 Category
Simple click an existing category from the Contents menu from the backend or create a new one and expand the Metadata Options panel on the right hand side of the screen.
Here you can set your desired meta description and meta keywords.
Extra features that have been added which are very nice include alternative page titles which will allow you to control the title of the page if you want it different to what is actually displayed on the content area of the screen. Author meta and the robots tag has also been included at the categories level which tells the search engine whether or not to index and follow content and links on the category level.
Add in the details that you need and hit Save & Close.