Chilli is a spicy spice, but how hot it is depends on the cultivar - some types are mild, while others can burn on contact with the skin. They also differ in spiciness according to the conditions in which the plant has been grown, how ripe the chilli fruit is when it is harvested and whether the seeds and membranes (the hottest parts of the chilli) are used. Dried chilli is much hotter than fresh because the flavour is concentrated.
Chilli is used in sauces and pastes where it's almost always mixed with other ingredients to help balance the spice level and give it a fuller, more complex flavour. Some cultivars are so hot that you don't even need to come into contact with the chilli - just smelling it can burn, as the capsaicin aromas irritate the mucous membranes.
Great care should be taken when handling fresh chillies. Try to touch only the waxy exterior and stem because the flesh, seeds and membranes contain capsaicin.
Dried chillies - whole or flakes - should feel slightly pliable, rather than brittle. They're often toasted, to bring out the flavour, but be warned that they burn easily, which makes them acrid.
Dried chilli powder can be single varietal (such as piment d'Espelette), but more often it's a blend, which helps producers maintain a consistent heat level. When buying powder, look for a bright colour; if it looks faded, it will have lost some of its heat. Store chilli powder in a cool place, such as a wine fridge or freezer.