Unconventional gas is a type of natural gas. Gas reserves are classified as conventional if they are found in the honeycombed rock layers (often in limestone or sandstone) which allow smooth gas flow through openings or by performing shallow drills. Unconventional resources are usually found in rocks with low permeability which makes them difficult to extract. This is mainly due to the special technologies that need to be applied in order to secure the borehole and increase bore flow. Reserves of this type of gas are usually enormous - much richer than traditional but also much more expensive and requiring significant expenses for determining where the resources are and whether their extraction will be profitable.
The popular term "shale gas" is merely a description of one type of natural gas extracted in an unconventional way. Differentiating them has fundamental value because each of them requires different methods of extraction and different technological solutions.