Valve will launch Steam China, a version of its platform and store in that country, and Perfect World will help. There's no release date yet and no indication of whether the service or content library will undergo changes to comply with Chinese internet and media regulations.
The press release announcing their partnership said that Valve and Perfect World will collaborate on the Steam China platform's promotion, but it's unclear if that collaboration extends to running the service itself. This is in keeping with China's restrictive rules, which only allow foreign companies to enter their domestic market if they've partnered with a local entity. The companies first paired up in 2012 when a Perfect World subsidiary licensed Valve's DOTA 2 and then CS:GO to sell in China, and it is expected to release more games through the forthcoming native version of Steam.
While the announcement noted that no changes are planned for Steam's "existing worldwide operations and services," it's unclear how the platform may operate (or what content it will hold) when it launches in China. Valve declined to comment further, except to say that more information will be made available "soon."