A shareholder agreement typically addresses board composition, which decisions need approval and by what majority, restrictions on transferring shares, what happens on death or departure, and how a deadlock is broken. It sits alongside the articles and bylaws rather than replacing them.
Why it matters: most serious founder disputes are not about whether a rule was fair, but about the absence of any agreed rule. Negotiating these terms while everyone is still on good terms is far cheaper than litigating them later.