Precision = the fraction of retrieved items that are relevant
(How much of what you retrieved is good?)
# relevant articles
# total articles
Recall = fraction of relevant items retrieved out of all relevant items available in the database
(How much of the good stuff did you actually get. Unfortunately, the higher the recall, the more 'junk' you end up getting also.)
# relevant articles retrieved
# of total relevant articles available
When searching, you're looking for a reasonable balance between precision (narrowing your search to get ONLY relevant articles) and recall (widening it to get ALL relevant articles, which usually means a lot more junk to weed through as well).
A common question is "How many articles should be retrieved by a good search?" There's no exact answer to that. Somewhere between 100-300 is a reasonable number of abstracts to weed through, but it depends greatly on your question, how comprehensive you want to be, and how much literature there truly is on your topic.