Section 16202(a) is the sentence that surprises people who have been running something together for years: the association of two or more persons to carry on as coowners a business for profit forms a partnership, whether or not the persons intend to form a partnership. Intention to create a legal relationship is irrelevant; what matters is what the parties actually did. There is no filing, no agreement and no name required.
Subdivision (b) excludes an association formed under another statute — a corporation, an LLC, a limited partnership — from being a partnership under this chapter. Subdivision (c) then supplies the rules for deciding whether the conduct amounts to co-ownership of a business for profit. Joint tenancy, tenancy in common, joint property, common property or part ownership does not by itself establish a partnership, even where the co-owners share the profits made by using the property; two people who own a rental flat together are not partners merely because of that. Sharing gross returns does not by itself establish one either.
Paragraph (c)(3) is the operative presumption: a person who receives a share of the profits of a business is presumed to be a partner, unless the profits were received in payment of a debt, as an independent contractor's fee or as wages or other compensation to an employee, as rent, as an annuity or retirement benefit to a beneficiary or representative of a deceased or retired partner, as interest or another charge on a loan even where the amount varies with profits, or as instalments for the sale of goodwill or other property. Share of net profits, rather than gross returns, is therefore the strongest single indicator. Whether a working arrangement has become a partnership — with the fiduciary duties in § 16404 and the joint and several liability in § 16306 attached — is a question of substance to take to a lawyer before anything goes wrong.