From Wikipedia: "Deep learning (deep machine learning, or deep structured learning, or hierarchical learning, or sometimes DL) is a branch of machine learning based on a set of algorithms that attempt to model high-level abstractions in data by using multiple processing layers with complex structures, or otherwise composed of multiple non-linear transformations."

So it refers to any architecture that has several layers. These layers might have various machine learning methods (even different ones). But usually this method is a neural network. (or RBM)

Neural networks with multple hidden layers are refered to as Deep Neural Networks. 

NN with complex layer is DL according to my knowledge.

NN has only two layers, while you can define as much as layers you want in DL.