What's the difference between .Xresources and .Xdefaults?

How are these two files connected to xrdb? Should I have both, or just one in my home directory? Do lines like these go in either file?

XTerm*background: white

URxvt.font: xft:Monospace

Xresources is a user-level configuration dotfile, typically located at ~/.Xresources

Using ~/.Xdefaults is deprecated, so this article will only refer to resources loaded with xrdb