हूँ is the first person singular form. It goes with मैं (I). Eg: मैं चाय पीता/पीती हूँ ('I drink tea'). पीता is used if you're male and पीती if you're female.

है is the third-person singular form. It is used when you're talking about someone other than the speaker and listener. Eg: 'नेहा चाय पीती है', 'पीटर चाय पीता है'. Again, पीता is used for a male subject and पीती for a female subject.