The Royal Tern is a large, orange-billed tern which is found only along ocean beaches.
Both a non-breeding adult and a juvenile have a white forehead and a narrow, shaggy band around the back of the head. 
A breeding adult has a complete black cap.
A juvenile Royal Tern is similar to a non-breeding adult, but its bill is smaller and pale yellow instead of orange.

Royal Terns look similar to Caspian Terns and they occur sometimes in the same place.
With these two species, the most important characteristics are the bill and the cap.
Caspian Terns are a little bit bigger and have a heavier and more red bill than Royal Terns which have a thinner and more orange bill.
Caspian Terns also have a dark tip on the bill that Royal Terns don't have.
Non-breeding Royal Terns have extensive white on the crown, whereas Caspian Terns are mostly dark on the top of the head with a small area of white on the forehead.
Royal Terns have the full black cap only from March to June, while Caspian Terns have it much longer (February to October).

In the company of Laughing Gulls. Flores, Petén, Guatemala, 20 January 2020