Searching begins from nothing and adds to a list of results based on criteria that matches. Filtering begins from the full list of results and eliminates from that list based on which results do not match certain criteria.
For example, let's say you wanted to find the Post "Richard Wilbur on what poetry is". When you enter "Richard Wilbur", "Poetry", or "Writing" in the search bar, the CMS will search from a list of Posts to see which ones have those Keywords. When the CMS searches for your entry, the search looks for the string of characters you type in, not the WORD. If you typed in "book", two possible search results include "John Steinbeck on finishing a book" as well as "The History and Sociology of Facebook".
In contrast, if you wanted to filter to find the post "Richard Wilbur on what poetry is", you would not enter anything in the search bar. Instead, you would pick out a Category such as "Language & Literature" and a Tag such as "Poetry". The CMS will then list the items under the Category "Language & Literature" and with the Tag "Poetry". It is important to note that you may only select one Category and Tag per search.