Keys are used to uniquely identify records in a table. it is used to establish relation within a table.
Primary keys,Foreign keys, candidate keys and alternate keys are used in tables which identify rows.

All keys other than primary key are Alternate key. For example Name,id,class,city,age. In this id is Primary key and rest are alternate keys.
If a table is having more than one candidate key, one of them is the primary key and rest of all are called alternate keys.
An alternate key is just a candidate key which is not selected as the primary key
any of the candidate key which is not part of primary key is called an alternate key

if we talk about alternate key, column may not be primary key but each column is unique. Primary key is essential with alternate key to identify records.

candidate key which are not selected for primary key are known as alternative keys
Alternate Key can be any of the Candidate Keys except for the Primary Key