What are OpenSim Archives (OAR)?

OpenSim Archives or OpenSimulator Archives (OAR) are files that can save all the necessary asset data of an entire sim so that you may fully restore the terrain, region parcel data, the textures of objects and their inventories when loaded onto a completely different system using a different asset database.
Source: opensimulator.org

Using OAR files educators and students can create, save, and share as many islands/sims as they want.