Need clarifications regarding the a role setup yaml

Below mentioned yaml file snippet is used for a role creation :
I am new in kubernetes and so looking for a reference link to elaborate the rules mentioned in the yaml . For example , my understanding is “” indicates core API groups of kubernetes and then my question is what is “extensions” for … Similarly for rest of the yaml looking for a reference / explanation. Thanks a lot guys for the help

- apiGroups: ["", "extensions", "apps"]

resources: ["*"]
verbs: ["*"]
- apiGroups: ["batch"]
- jobs
- cronjobs
verbs: ["*"]

“” indicates core API groups of kubernetes

Cluster information:

Kubernetes version:1.12
Cloud being used: (put bare-metal if not on a public cloud) : Azure

Have you checked:

I think it is explained there, IIRC. But please do ask if something is not clear