I love the metaphors (or similes) that instructional librarians use to get points across to our students about some of the more complex ideas. Here's another great one recently posted to the CJC-L:
On a related note, I like to tell students Academic Search Complete and Proquest Research Library are like Wal-mart and Target. They have multiple departments and there is overlap between the two but there are some items you can only buy at Wal-mart and some only carried by Target. The subject-specific databases are like the shoe stores, sports stores, bookstores, etc. you find at the mall that cater to consumers looking for a particular item. (-Christina Teasley, Savannah Technical College)
If you have any of these great metaphors, please share them. I think it makes our classes more interesting and understandable when we get away from "library-speak" and make our concepts relevant to students' "prior knowledge" (to get all pedagogical on you!). One of the first ones Julie shared with me was using "hurricane" to explain the lack of context when keywording search (i.e. the word "hurricane" could refer to a storm, a lamp, a boxer, a movie, a Bob Dylan song, an alcoholic bevarage, an NHL hockey team, etc). I still use that in English I classes (thanks Julie!!).