* Noun: This is the most common usage, referring to a person who receives guests, or the person who presents a television show or radio program. Example: "The host welcomed everyone to the party."
* Verb: This usage means to provide accommodation or to organize an event. Example: "The hotel hosted a conference."
* Adjective: This usage means "to be present in a living organism" (usually used in reference to bacteria or viruses). Example: "The bacteria are host to a virus."
To determine the part of speech, you need to look at the word's function in the sentence.