Verb: This is the most common use. It means to refuse to give something, or to keep something back.
Example: "The company withheld his salary until he completed the project."
Adjective: This use is less common and usually refers to something that is held back or kept secret.
Example: "The information was withheld from the public due to security concerns."
Therefore, the part of speech for "withhold" depends on its context in a sentence.