The LLC is typically the best choice for smaller entities. The LLC structure provides a great deal of ownership flexibility in that an LLC may have any number of Members (owners) including non-US citizens and subsidiary companies. LLCs are also able to distribute several different classes of stock or ownership interest. However, their owners are typically required to pay a self-employment tax.
A series LLC is a form of limited liability company that provides liability protection to multiple "series". Essentially, it's a master LLC with separate divisions, each protected and operating independently. As an entity, the series LLC is geared towards businesses where investors own multiple companies, with each series being protected from the debts and obligations of the other series. Currently, only several states support this option, including Delaware, Illinois, Iowa, Nevada, Oklahoma, Puerto Rico, Tennessee, Texas, and Utah.
A partnership is a business relationship entered into by a formal agreement between two or more persons or corporations carrying on a business in common. The capital for a partnership is provided by the partners who are liable for the total debts of the firms and who share the profits and losses of the business concern according to the terms of the partnership agreement.