Neoplasia is new, uncontrolled growth of cells that is not under physiologic control. A "tumor" or "mass lesion" is simply a "growth" or "enlargement" which may not be neoplastic (such as a granuloma). The term "cancer" implies malignancy, but neoplasms can be subclassified as either benign or malignant.
There is no single mechanism by which a neoplasm arises. Many different mechanisms give rise to neoplasms, and that is what makes diagnosis and treatment so challenging.