A good way to think about both benign and malignant neoplasms of the skin is to think that every normal constituent of the skin has potential for either benign or malignant growth. Therefore, hair follicles get benign and malignant tumors, as do blood vessels and nerve tissue and sweat glands. And, of course, keratinocytes. Well, here's the dealio on glomus tumors. They are benign growths of the glomus bodies which shunt blood to the peripheral extremities, so as to thermoregulate.
They tend to be solitary, although some people get them multiply, they tend to be tender (this patient yelped when I pressed on it) and they have that bluish subcutaneous nodular look, as in the picture above.
The main ddx on a tender nodule on the digits would be an Osler's node, but Osler's nodes are infective vasculitis, and as such tend to look and be necrotic in nature, rather than as above, which looks more like a tumor. Think inflamed versus non-inflamed.