Lemon grass is used to add a citrus flavour to Thai dishes. It is especially useful for soups, whereas lemon juice would lose its fragrance during cooking, the lemon grass flavour strengthens as it cooks. It is available from Asian grocers either in fresh form, or in dried form. Get the fresh form if you can.
It requires no special preparation, simply chop and add to a dish.
Dried lemon grass should be kept in an airtight container.
Fresh lemon grass can be kept in a deep freezer for 3-5 months.