For filling small moulds, making small fancy items, shaping small cakes.
If you intend to buy:
The bag can be a nylon reusable one (washable), or single use (sold in multiple packs).
Nozzles (or icing tubes) are available in 2 different materials: metal (old fashioned, less practical) and polycarbonate (very hard plastic, more practical). There are two main types of nozzle: star (toothed or ribbed) and plain (smooth). Star nozzles are used for decorative effects. And finally nozzle diameters, usually given in millimeters: I advise you to get a set of several diameters (from 4 to 12 millimeters for example).