Brown is suing CEO Evan Spiegel and CTO Bobby Murphy for one third of the super-hot company, which is valued at $800 million.
The story is complicated, as lawsuits tend to be. Basically, Brown was intimately involved with the founders of Snapchat right at the beginning. How much work he actually did for the company is in dispute.
Later, when Brown realized Spiegel and Murphy did not recognize that he had any ownership of the company, he sent them this email (below), which details a fight between the three, ending when Brown's internal passwords were changed — locking him out of the company.
At that point, Brown talked to a lawyer and discovered that his early involvement, and the patent filing, gave him a claim to a third of the equity in Snapchat.
The other founders seem to regard him as a former employee to whom they made no such promises.