Visa not required
This is also known as “visa-free entry” which entitles a certain passport holder privilege to enter the country for a specific period in what is usually termed “landing permssion”

Visa on arrival
This means a certain passport holder still needs a visa to enter but can apply for it and receive the visa upon arrival at the airport.

An e visa or electronic travel authority is a tourist and travel visa which comes in electronic format,meaning no label or stamp ever appears in your passport.