Wilton calls them tips, Ateco calls them tubes. I think "tube" is a bit of a misnomer; it suggests something cylindrical, whereas "tip" describes where on the decorating bag it's located. In a short span of time I've amassed a vast array of tips, mostly because they tend to run pretty cheap (between $1 and $3). Some of the standouts pictured in the multi-tip pileup above include one that pipes 3 stars simultaneously (far left; ideal for covering large areas with stars), a wide icer tip (that toothy monster toward the middle; I prefer to ice cakes with an offset spatula but the icer tip gets the job done), a Christmas tree tip (ideal for piping a cute little detail when you don't have the time to draw something freehand), and a long cupcake filler tip (to the right of the tree, standing almost as tall as the wide icer tip; get two or more of these and you can inject as many fillings as the cupcake will hold, such as peanut butter and jelly).
Invariably when I go shopping for tips I get carried away. I would wind up with unintentional duplicates of tips if I didn't carry lists of those I have (by brand) in the "Notes" app on my phone. I like to load up a decorating bag and take to my practice board with the new tips as soon as possible, always including the number of the tip…
Ateco Cake Decorating Reference Manual, which is pretty much nothing more than 20 pages chock-full of images like the one above. All of their examples are shown life-size, and in white icing. I think white icing does look more palatable in photos than brown, but soon I'll post the reason why I happened to have a decorating bag loaded with chocolate icing (and the 195 tip) lying around.
I hope 2012 is off to a great start for you!