IMO, all criminal defendants are hostile parties to some degree.
A hostile witness, however, is someone whose testimony is adverse/ to the lawyer who is questioning the witness. That is, a witness is hostile when his testimony is not helpful to the lawyer’s questions. The witness may be prejudiced against the party that lawyer represents.
When a witness is declared hostile, the questioning lawyer is allowed to ask leading questions on direct examination. Leading questions prompts or encourages certain answers, while direct questions cannot be in a form that suggests an answer to the question.
Direct questions are more open and can begin with who, what, where, when, and how. A leading question could be, for example, “Isn’t it true, Mr. Thompson, that PF asked you many times to (kill) his fiancé?”