Simply, a knowledge base is a list of articles displayed and grouped by their categories. To help the users to find quickly their answers, a search engine is oftenly used and displayed on the top of this kind of page.
Once we've defined this, it's easier to understand how we'll create this knowledge base just with Joomla. We'll have to create:
- the categories (and sub-categories if needed)
- the articles in each categories
- and the search module
To manage your knowledge base easily, I suggest you to create first a parent category named "Knowledge base" and create as many child categories you need.
Add a short description to your categories.