The "We Are Still In" campaign is more of a statement than an agreement for concrete action or cooperation. From the press release that they issued, it's basically to "declare that we (they) will continue to support climate action to meet the Paris Agreement":
The statement calls “The Trump administration’s announcement [one that] undermines a key pillar in the fight against climate change [and a move which is] out of step with what is happening in the United States.” The signers all understand that the Paris Agreement is a blueprint for job creation, stability and global prosperity and that accelerating the United States’ clean energy transition is an opportunity - not a liability - to create jobs, spur innovation, promote trade and ensure American competitiveness. By declaring that “we are still in,” the signatories are putting the best interests of their constituents, customers, students and communities first while assuring the rest of the world that American leadership on climate change extends well beyond the federal government.
However, the agreement that California signed with China is a Memorandum of Understanding (MoU) which expands cooperation and establishes a partnership with China. It will also establish a joint team of Chinese and Californian officials to come up with ways for cooperation and to invest in programs that would cut carbon emissions.
This is an excerpt of the MoU regarding its objective:
The objective of this Memorandum of Understanding (“MOU”) is to support the Participants in advancing innovation and development of, and investment in, low-carbon energy resources and clean technologies. Cooperation between the Participants will be based on a mutual understanding of shared issues and concerns of the governments and citizens of California and the People’s Republic of China as they relate to research, innovation, and commercialization of clean technologies.
The full MoU is available in a press release from Governor Brown's office.