Doors nowadays can be made from various materials including wood, steel, fiberglass, UPVC/vinyl, aluminum or glass.
Doors can often be recycled, especially those made from wood.
Check your local civic amenity site on the Waste Services Locator on this site.
Some local authorities may also provide a bulky goods collection service for a fee.
If you have a door that is still usable, you could consider selling it or passing it on to someone who needs it.