Difference between PBR and Shaders?

I’m confused between PBR and shaders,can you tell what PBR does and what shaders do?

PBR is just a way for shaders to be calculated, they’re not the same kind of thing.
A simple way of explaining shaders according to wikipedia: “a program that tells a computer how to draw something in a specific and unique way”
The more traditional way of making shaders used diffuse and specular to “fake” the desired result while PBR - Physically Based Rendering is a closer representation of how light actually works in real life.
PBR usually works a lot better in varying light conditions, while dif/spec at times could look bad or different once the light changed.

A bit more of info: I'm kinda confused between PBR and Deferred shading? - General Discussion - Unreal Engine Forums