Cardamom is a seed pod from one of two types of related herb. They both have very different flavours, with the black cardamom being used in curries for its distinct strong, bitter flavour, and the green cardamom being used in lighter curries and even sweets for its delicate, fragrant flavour. Both black and green cardamoms are usually supplied in pod form for longevity, though are often crushed just prior to use to bring out the flavours.
Green Cardamom is probably the most familiar of the two. Its distinct flavour is often used in Kulfi and other Indian sweets. In addition, green cardamom is often thrown into curries. It has a light flavour, and adds a slightly flowery fragrance. You can add the pods whole to a curry, but it is usually better to crush them with a rolling pin or mortar and pestle before use, to release the flavours. Each pod contains several little tiny seeds.
Black Cardamom is less familiar to most. It is a very strong cardamom, and is usually added into curries cooked in large pots. You only need a few. Black Cardamom is also a common constituent of garam masala, where it is crushed together with the other spices to make the familiar blend. You can also pop a few whole black cardamoms into rice, where the boiling process really releases the flavours into the rice.