В чем разница между ключом-кандидатом и составным ключом?

I am reading about candidate keys and composite keys. I came to know that

• a candidate key can qualify as a primary key and it can be a single column or combination of columns
• and a composite key is also a combination of columns.

For composite key I have referred this link

how do I make a composite key with SQL Server Management Studio?

Thus when both the candidate key and composite key are a combination of columns, they can qualify as a primary key. Then what is the exact difference ? Can you please explain with examples ?