Chillies can be red, green orange or yellow. Generally the longer chillies are not as hot as shorter chillies. Red chillies are hotter than green chillies. There are many varieties of chillies grown throughout the world with different cuisines using different varieties. Much of the heat of a chilli is in the seeds as well as some bitterness, so remove seeds if you desire less heat.
The main types available in Australia are:
Are fairly hot, small, chillies that are red when ripe. They are used in Asian cusine.
Long green and red Chillies are less hot and good in sandwiches, pickled or used raw or cooked in curries and stirfries.
Jalapeños are hot, small, dark green chillies used in Mexican cuisine.
Cayenne chillies are very hot, long, narrow, red chillies which are dried and made into cayenne pepper.
Habañeros are extremely hot, orange or red pods found in many commercial hot sauces.