Public Relation or Public Relations

[new2grammar]

In general wqhenever we speak to someone we say public relation, but whenever i read articles in newspaper, online it is public relations. I just need the explianation which one is correct public relation or relations, or we can use either of them. And if relations is correct than please explain why 's' is mandatory and difference it makes.

Thanks in advance!

[Anglika]

If you are talking about promotional matters, it is always Public Relations.