What kind of license we need to offer song downloads on a website

I have seen many many websites which allow you to download latest songs for free I am doubtful that they have any kind of license or partnership with any music company so if they do not have any such thing how come they are running websites with thousands of pages indexed on google. If they have any license or partnership for the same what it is? And how they get all the songs on their website from the very first day of release. I am running a website where people can listen songs online but I am using soundcloud API for that.

I am asking this question over here because I do not know why but I cannot ask question on stackoverflow.com and this question doesn't seem suitable for music community of stackexchange group.