Tumors and cysts are two different things. Specifically:
A cyst is a "sac" that may be filled with air, fluid, or other substances. Cysts can appear anywhere in the body, including bones, organs, and soft tissues. The vast majority of cysts are benign and do not become cancerous. However, nearly all cancers can cause cysts.
A tumor is an abnormal growth of tissue. Like cysts, tumors can occur in any part of the body. Some tumors are benign, while others are malignant (i.e., cancerous).
To determine whether a cyst or tumor is benign or malignant, a biopsy is usually needed. This involves taking a sample of the abnormal tissue, or in some cases, removing the entire suspicious tissue, and then examining and evaluating it under a microscope.