I'm not what you'd call a huge fan of drop flowers; there's something so prefab about them. Basically, you hold a tip perpendicular to whatever surface you're going to pipe on, squeeze out a flower while either holding the bag still or rotating it, and then add one or more dots or a star to the center. They don't have a lot of individuality. However, they're easy to crank out, and if you need 350 icing flowers to festoon an old-school wedding cake, they're probably the right flower for you unless you have nothing but time on your hands.
The Wilton book from 1986, The Uses of the Most Popular Decorating Tips, has an amusing passage that might lead one to believe that some sort of snobbery about drop flower tips exists: "True drop flower tips have a center rod within the cone-shaped tip. This pipes a ring of petals with an open center." Be that as it may, you can pipe drop flowers with regular star tips, and I don't think the effect is different enough to be noteworthy. Here are three "true drop flower tips" though, according to the Wilton book:brush or one like it).
When you attach the tip to the coupler, line up the nail with that notch on the side, and it won't rotate around. Now you know what that notch is for, if you didn't already.
…and these were piped with a twist.
These were piped with an Ateco #96:
These were piped with a Wilton #224:
These larger flowers were piped with a Magic Tip #191:
And lastly, these tiny daisy-like flowers were piped with a Wilton #16, a small star tip rotated…
…and held straight.
You can pipe drop flowers in buttercream directly on a cake, but I prefer piping them in royal icing on parchment, letting them dry, and then relocating them (or piping them directly on cookies, of course).