Piping bags are used to fill canapes, as well as drizzle everything from chocolate sauce to whipped cream and even savory pastes. Disposable piping bags are made of heavy plastic. I have two cloth ones from cooking school that get washed and re-used. Make sure to air dry sufficiently or you get mold or stinky smells!
If you forgot to buy or bring a piping bag to a catered event, you may be able to use a baggie. Of course, there's a catch: the baggie only works if the material you are piping is more liquid than paste. Think mayo, pesto, that sort of thing. To make your own piping bag, use a one gallon sturdy baggie. Cut the bottom corner off according to how wide you need your piped liquid to be: one quarter inch is a great measurement to start with. Don't make the hole too big or the liquid will rush out too quick and be too wide a ribbon.
If you are piping a thick dip, the plastic bag will start to tear in little slits. A baggie is not sturdy enough to handle the pressure of squeezing out thicker substances. Trying to force a baggie to work, almost guarantees you'll have a mess of oozing multiple holes. Not the best way to get things done or impress your clients and boss!