What does NADH dehydrogenase do?

Hint: Almost all enzymes are proteins that catalyse the biochemical reactions in living cells, hence are called biocatalysts. Enzymes are proteinaceous in nature with the exception of recently discovered two RNA enzymes named as Ribozyme isolated from Tetrahymena and Ribonuclease discovered from bacteria.

Complete answer:
An enzyme like any other protein has secondary and tertiary structure. In the tertiary structure the backbone of the protein chain folds upon itself, the chain criss-crosses itself and many crevices or pockets are made. Such pockets are called 'active sites'. An active site of an enzyme is a crevice or pocket into which the substrate fits. A substrate is a specific compound and acted upon by an enzyme. Enzymes are organic catalysts, there are inorganic catalysts also, which do not occur in living cells.
Purine and pyrimidine nucleotides polymerize to form nucleic acids. Higher purine and pyrimidine nucleotides, particularly ATP, store energy in their high energy phosphate bonds. They are formed during photosynthesis and respiration. Hydrolysis of the phosphate bonds of ATP releases their bond energy for driving energy-dependent reactions and processes. Nicotinamide and riboflavin nucleotides act as coenzymes of oxidizing enzymes. NADH dehydrogenase is an enzyme that converts nicotinamide adenine dinucleotide (NAD) from its reduced form (NADH) to its oxidized form ($NAD^+$).
Enzymes are composed of one or several polypeptide chains but many enzymes exhibit the catalytic activity only in association with certain non-protein substance. Such substances are called cofactors. In these enzymes the protein portion is known as apoenzyme.
Three types of cofactors are identified:
(i) Prosthetic group- They are tightly bound to the apoenzyme. For example haem is the prosthetic group in peroxidase and catalase enzymes that catalyse the breakdown of hydrogen peroxide to water and oxygen.
(ii) Coenzymes- The essential chemical components of many coenzymes are vitamins e.g., coenzyme nicotinamide adenine dinucleotide (NAD) and NADP contain the vitamin niacin.
(iii) Metal ions - Example is Zinc as it is a cofactor for the proteolytic enzyme carboxypeptidase.

Note:
Properties of enzymes include proteinaceous nature; they do not begin the chemical reaction but increase the rate of reaction. Enzymes are in no way transformed or used up in the chemical reaction but come out unchanged at the end of reaction. They are highly specific in their actions.