A mnemonic device is "any technique used for the purpose of either assisting in the memorizing of specific material or improving the function of memory in general." There are several types of mnemonic devices. You've probably used them before without even realizing! Most people use acronyms, acrostics, and rhymes, but there's also grouping, visual association, and the Method of Loci in which you place the items you want to remember in a visualized room or route.
The concept of mnemonic devices is a helpful one, especially to college students. Everyday we are presented with loads of information that we need to absorb so that it can be used at a later date. Learning mnemonic devices can help us with that. Take this video for example. In the popular TV show The Office, we see Michael Scott use mnemonic devices to remember his clients names. Although his way of doing so is offensive, it does work!
Mnemonic devices can be used for many things: remembering dates, remembering names, remembering the order of the planets (I'm sure as a child you were taught My Very Educated Mother Just Served Us Nachos!); the list is endless. I'm sure you also learned this one, too: 
However, mnemonic devices don't work for everything. Although they are easy to use for recalling small pieces of information, or simple facts, larger concepts and ideas are a lot more difficult and many times making the mnemonic device more difficult to come up with and memorize. When this happens, it is important that people learn to recall the information in other ways.
No no no... it's My Very Eager Mother Just Served Us Nine Pizzas... You really never learned Pluto as a planet? That makes me feel old. :) Good post. Mnemonics helped me pass many classes.