What's the difference between a primary key and a unique(Candidate) key

uniqueness

Both primary key and unique key enforces uniqueness of the column on which they are defined.

Indexes

• primary key creates a clustered index on the column
• unique creates a nonclustered index by default

Nulls

•  primary key doesn't allow NULLs
•  but unique key allows one NULL only.

Multiple

• you can have multiple unique keys in a table
• can have only one Primary Key

Both of them can be refered as Foreign Key

A foreign key is a column (the child collumn) in a table which has a corresponding relationship and a dependency on another collumn (the parent collumn) that is usually in a different table.