Nodules are solid, elevated lesions that generally reach deep into the dermis and subcutaneous tissue. They are generally 1 - 2 cm in diameter (so think of them as going as deep as 2 cm into the skin). The lesions may be filled with infectious infiltrates, hyperplastic cellular elements, or other metabolic deposits.
When the lesion goes deeper than 2 cm, it should be called a tumor (mass). This is not to imply that other types of lesions are not "cancerous". Meaning, a papule or nodule can be cancerous. The term "tumor" vs "nodule" sometimes leads to this misunderstanding.