Once decay is removed from a tooth, a cavity remains which needs filling up with something in order that food doesn’t stick in the cavity left behind. Different materials can be used – the strongest being dental amalgam, but white fillings, made from types of tough plastic can also be used when indicated, though these are generally not as strong in locations where they are subject to loading during chewing e.g. on back teeth.
When a cavity is too large to hold a filling securely, then a crown may be placed. A crown is technically called a “Full Jacket Crown”, and sits like an external support around the inner tooth and core – just like a “bucket on a sandcastle”.