Capitalized:
* When referring to a specific group of people: "The Staff at the library is very helpful." Here, "Staff" refers to a specific group of people who work at the library.
* When used as part of a formal title: "Staff Sergeant Jones"
* In some company names: "Staff Solutions"
Not Capitalized:
* When referring to the general concept of employees: "The company is hiring new staff."
* When used as a synonym for "workers" or "employees": "The staff worked late to finish the project."
In general, it's best to err on the side of not capitalizing "staff" unless it's part of a specific title or referring to a particular group of people.