Citrus zest is one of the most powerful and distinctive seasonings you can use but most people just discard it.
To make the most of it, you need to separate the zest from the pith, which together compose the citrus peel (also called the rind or the skin). The zest is the colourful part of the peel: the thin layer of yellow, orange or green on a lemon, orange or lime, while the pith is the thicker white layer underneath the zest. The zest has a bright taste and aroma due to the citrus oils it contains while the pith has a cottony texture and bitter flavour.
There are several methods of varying efficiency for removing the zest from the peel. A very sharp vegetable peeler will take off the zest in wide strips, although it also usually removes a layer of the pith, which needs to be scraped away. This size is good when you want to infuse the zest in liquid but also remove it easily (you can just fish it out of a mulled wine, for instance). A citrus channel knife (also called a "stripper") works better at taking off the zest in long strands that are about 3mm wide; this is the tool most bartenders use to create a lemon (or lime) twist for a drink. A citrus zester - a tool with five sharpened holes - takes off the peel in long, fine strands, which look pretty when used to decorate desserts. A microplane-type rasp grater creates a light, cloud-like drift of very finely grated zest. I use the rasp grater when I want zest to use as a seasoning for dishes: I mix it into marinades, salads and desserts.
Dried zest is useful for flavouring food. It's best to start with large strips of zest because it shrinks as it dries. Remove the zest with a peeler, channel knife or zester, then lay it out in the sun for a few days, turning it over occasionally. If it still seems damp, put it in a low oven (about 60 degrees Celsius) until it's dry. Wrap it then store it in the freezer, or make citrus salt or sugar by finely chopping the zest then grinding it with salt or sugar in a food processor or spice blender.