Most common is a fireplace, almost every Portuguese house has one, even apartments. Firewood is sold in supermarkets and specialised shops on main roads from December to February.
Also very popular is heating by hot air through the air conditioning system. This is a very cost effective and environmentally friendly way of heating, plus it is fast. When coming home after dinner, you probably don't go through the hassle of firing up the fireplace whereas the aircon will heat a room in 5 minutes.
Also radiator heaters are very fast and are available from 30 Euros, although they tend to use quite a lot of electricity.
You will find under-floor heating in some modern and upscale houses, especially in bathrooms.