From the website:

Croquet is a powerful new open source software development environment for creating and deploying deeply collaborative multi-user online applications and metaverses on and across multiple operating systems and devices. Derived from Squeak, it features a peer-based network architecture that supports communication, collaboration, resource sharing, and synchronous computation between multiple users on multiple devices.

Every part of the system is designed around enabling real-time, identical interactions between groups of users. Croquet's architecture is designed to make it easy to develop deeply collaborative applications without having to spend a lot of effort and expertise in understanding how replicated applications work.

Using the Croquet Software Developer's Kit (SDK), developers can create and link powerful and highly collaborative cross-platform multi-user 2D and 3D applications and simulations - making possible the distributed deployment of very large scale, richly featured and interlinked virtual spaces.

From the website:

Croquet is a powerful open source software development environment for the creation and large-scale distributed deployment of multi-user virtual 3D applications and metaverses that are (1) persistent (2) deeply collaborative, (3) interconnected and (4) interoperable. The Croquet architecture supports synchronous communication, collaboration, resource sharing and computation among large numbers of users on multiple platforms and multiple devices.