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.