The Crownstone SDK exists of three parts (in an increasing order of integration):
☁️ A REST API in the cloud
📱 Smartphone libraries
👑 The firmware on the Crownstones, called bluenet.
💻 A hub to bridge a Crownstone mesh to the cloud.
The cloud is required to setup the Crownstones: keys and IDs will be generated, and locations can be set. After that, it can be used to add users, so they can also make use of your Crownstones. The cloud is also used to synchronize data between users, and serves as data storage. You can read how to use it in the REST API documentation.
You can read more detailed cloud documentation here.
The documentation on the smartphone application can be found here.
An entry to the documentation of the firmware can be found here.
An entry to the documentation of the hub can be found here.