A limited liability company, or "LLC", is an unincorporated business entity which can be called a cross between a corporation and a partnership. Like a corporation, an LLC protects its members from personal liability for the debts and obligations of the company. Like a partnership, an LLC is typically formed by filing a "certificate of formation" or similar certificate with the Secretary of State and is taxed like a partnership. Also like a partnership, the members of LLCs typically enter into an operating agreement which establishes how the LLC is managed.  This agreement controls the management of the company and how the members relate to each other.

Where S Corporations have limits on the number of shareholders who also must be US residents, LLCs have no restrictions in these regards.  This makes the LLC a particularly suitable vehicle for non-US residents.  An LLC can have more flexibility in management because this is controlled by the members agreement not by the Business Corporation Act (BCA) of the state.

Unless the LLC elects to be taxed as a corporation, it will be taxed as a partnership - income and deductions of the LLC will be passed through to members for inclusion in their personal income tax returns.  See How will my Company be Taxed?  for more information.

Bottom Line:
If one or more of the owners are US non-residents or if you have a non-traditional management structure and so need more flexibility than the standard Officers and Directors arrangement of corporations governed by the state's Business Corporation Act provides, then an LLC may be for you.  If tax considerations are a driving factor, you can achieve the same pass-through taxation by electing S Corporation status as a corporation.