Nerve growth factor is a small secreted protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 that is important for the growth, maintenance, and survival of certain target neurons (nerve cells). It also functions as a signaling molecule. It is perhaps the prototypical growth factor

Growth factor

A growth factor is a naturally occurring substance capable of stimulating cellular growth, proliferation and cellular differentiation. Usually it is a protein or a steroid hormone. Growth factors are important for regulating a variety of cellular processes....

, in that it is one of the first to be described. While "nerve growth factor" refers to a single factor, "nerve growth factors" refers to a family of factors also known as neurotrophin

Neurotrophin

Neurotrophins are a family of proteins that induce the survival, development, and function of neurons.They belong to a class of growth factors, secreted proteins that are capable of signaling particular cells to survive, differentiate, or grow. Growth factors such as neurotrophins that promote the...

s.