What is dimensionality reduction? What is the difference between feature selection and extraction?
Simply put: 


feature selection: you select a subset of the original feature set; while
feature extraction: you build a new set of features from the original feature set. 


Examples of feature extraction: extraction of contours in images, extraction of digrams from a text, extraction of phonemes from recording of spoken text, etc.

Feature extraction involves a transformation of the features, which often is not reversible because some information is lost in the process of dimensionality reduction.