Diffuse - specifies how much/how model will be lit when the model receives light. (how much light should it reflect and absorb)
Ambient - specifies how much/how model will be lit when the model does not receive light. (Indirect light, the model would be lit by a uniform colour all over.)
Specular - when light strikes a smooth surface, they generally reflect in a sharp direction. This forms a specular highlight and it is viewport dependent. I think it is harder for me to explain you this because i suck at explanation. I think you should refer to a book, I'd suggest Frank D. Luna's DX11 book.