Cracked iPhone 4 screen! What is a digitizer and what is a lcd?

Cracked only my screen, still responds perfectly fine to touch. But I just want a new screen. However I have no idea what a 'digitizer' or an 'lcd' is? What do I need? Can I just buy the glass outer screen and replace it that way?

The digitizer is the entire assembly the outer screen and then digitizer layer and the support bezel underneath… Buy the entire digitizer assembly because your not going to be able remove the cracked screen from the digitizer without destroying the film underneath