A tumor is any abnormal mass of undifferentiated cells within or on a multicellular organism. They can be nothing more than a superficial oddity, or they may interfere with vital body functions. Large tumors may block veins and tracts, compress internal organs, and absorb nutrients needed elsewhere.
Tumors may be benign, which means that they will generally grow much more slowly (or grow to a certain size and stop) and stay put in one area.
Other tumors can be malignant (cancerous), which means they will usually grow rapidly and spread to other parts of the body (usually via the lymphatic system or the bloodstream) or cause more tumors to form elsewhere (this process is called metastasis). Melanomas and some types of breast cancer can spread and kill with blood-chilling speed. However, some malignant tumors, such as carcinoids and many prostate cancers, can grow very slowly and take years to cause illness.
To correct the existing writeup in this node, the cells in tumors do not grow uncontrollably unless they are cancerous. People come down with many benign tumors that cause no harm and don't spread. For instance, raised moles on one's skin are technically tumors, but they are so commonplace that no one thinks much about them unless they are oddly-shaped or large and warrant removal for cosmetic reasons or because they show signs of turning malignant.
Tumors develop because cells stop growing as they should. This may be due to inborn or acquired genetic damage (for instance, due to exposure to mutagenic chemicals or radiation) or from chemical irritation that leaves the DNA intact but causes the cellular machinery to go awry.