An Object is any part of Workforce.com that can contain information.
Shifts, Employee Profiles, and Team Profiles are kinds of objects. Every shift has a time, date, employee name, team, and location. Every Employee Profile has a name, the pay details, qualifications, and so on. Every Team Profile has a name, the employees within that team, tags, etc. With The Platform, you can create your own object.
Custom Objects adds more customization to Custom Fields by letting you create your own object. This is part of The Workforce.com Platform.