There are countless photo competitions listed on the Internet, all easy and quick to find using a search engine. Most are open to both amateurs and professionals. Themes are extremely varied and virtually unlimited. Sometimes clearly defined such as nature photography, sometimes just general, or with a variety of topics plus a general category. Depending on the competition organizer, entries are usually judged by a jury of experienced, high-profile photographers. However this is not always the case.
A photo competition can be a good guide to where you stand with your photography in comparison to other photographers. To get an idea of this you need to choose a competition that gives you some feedback. Unfortunately, even some quite high-level competitions simply return photographs stating that your entry was not accepted and wishing you “good luck next time”. There are others however that provide clear information, often in the form of marks or even keywords as pointers. Prizes vary greatly, sometimes consisting of cash prizes and very often cameras and photographic equipment.