Pyruvate is a three carbon molecule produced by the splitting of glucose in the cytoplasm of a cell. Under aerobic conditions, the pyruvate enters the mitochondria, where is it modified by a three-part enzyme complex known as pyruvate dehydrogenase. The pyruvate is converted into the two carbon molecule called acetyl Co-A by removal of one carbon, which is joined to an oxygen molecule to form a molecule of carbon dioxide. The NAD+ receives a electron which is freed during this step of the reaction, becoming NADH. (NAD+ has an H+ ion bound to it, which is where the electron attaches, hence the name change from NAD to NADH - the H was there the whole time.) The NADH then shuttles the electrons to the electron transport chain, where their energy is harvested by the cell; the electrons eventually are used to reduce O2 to create water. When the NADH gives the electron to the transport chain, it becomes NAD+ once again, and is reused; it may help you to think of NAD's role as that of a simple transport vehicle.
pyruvate converted into acetyl-CoA in the presence of pyruvate dehydrogenase enzyme in this reaction pyruvate is oxidized so NAD can accept H from it and reduced itself.