Difference between 15 vs 15A Receptacle and Wire size

Hello all, On homedepot website, the receptacle is of 15 or 15A. Question 1> What is the difference between 15 and 15A? What is better for residential usage? Question 2> How do I know the correct wire size allowed for the receptacle? For example: Leviton 15 Amp Duplex Outlet - White-R52-05320-00W - The Home Depot (http://www.homedepot.com/p/Leviton-15-Amp-Duplex-Outlet-White-R52-05320-00W/202066670)