A “stemcell” is a customized operating system image containing the filesystem for BOSH-managed virtual machines.
Stemcells provide a powerful separation between the OS and the other software packages bundled in a deployment. Each stemcell - no matter the underlying infrastructure - is exactly the same. This allows for rapid, reliable mobility between different infrastructure targets. Stemcells are key to the application portability delivered by Pivotal Cloud Foundry (PCF).
Similarly, Pivotal regularly publishes tiles for PCF tested against the latest stemcells to remedy identified vulnerabilities.
Read the bosh.io documentation - Linux stemcells
Read the Pivotal documentation - Windows Server 2016 stemcells
Read the Pivotal documentation - Windows Server 2012R2 stemcells