NAD+ is the abbreviated symbol for Nicotinamide Adenine Dinucleotide. It is a coenzyme present in biological systems. NAD+ is involved in the metabolism specifically in redox reactions. It accepts electrons from other molecules and become reduced.
NADH is the reduced form of NAD+. Similar to NAD+, NADH also participates in metabolic processes when reduction is necessary. When NADH reduces a compound, it gets oxidized to NAD+.