A contributor is someone who takes part in something or makes a contribution. Writers and people who donate money in particular are called contributors.
Writers whose work is included in a book, magazine, or newspaper are called contributors. Some have specific titles like "contributing editor" or "contributing writer." If you give money for a cause or a company, that contribution also makes you a contributor. This word is used mainly for writing and donating, but it can be used in other cases for people who help with something. The surest way not to be called a contributor is to do nothing: contributors have to contribute.