Biological molecules such as carbohydrates possess a great deal of stereochemical information so it becomes time-consuming to draw all of the stereocenters in the "wedge-dash" format. Emil Fischer (Nobel Prize in Chemistry 1902) introduced a simple and fast way to represent such molecules in so-called Fischer projections. The basic definitions are laid out below; the molecule is orientated so that the carbon framework is vertical, the horizontal bonds should then be coming towards you and the vertical away. Once this alignment is produced, the Fischer depiction is drawn as shown with the understanding that the horizontal lines represent wedges and the vertical ines represent dashes. Try to manipulate the middle molecule, which has one chiral center, so it matches the wedge-dash picture on the left, and then the Fischer depiction shown on the right.
Through some extremely clever experiments and logic, and without the use of modern tools such as NMR spectroscopy, Fischer was able to work out the overall stereochemical structure of D-(+)-Glucose (shown below). In the acyclic form there are 4 stereocenters, shown below in wedge-dash, moveable, and Fischer projection formats, which leads to 16 stereoisomers being possible (2n = 16).
Note: grey = C; white = H; red = O