What do you understand by the terms candidate key and alternate key in a relational database?

All attribute combinations inside a relation that can serve as primary key are candidate keys as they are candidates for the primary key position.

A candidate key that is not the primary key is called an alternate key. In other words, any attribute that is a candidate for the primary key, i.e., which is capable of becoming a primary key but is not a primary key, is an alternate key.