What is a container in YARN?

What is a container in YARN? Is it same as the child JVM in which the tasks on the nodemanager run or is it different?

It represents a resource (memory) on a single node at a given cluster.
A container is

• supervised by the node manager
• scheduled by the resource manager

One MR task runs in such container(s).

There can be multiple containers on a single Node (or a single very big one).

Every node in the system is considered to be composed of multiple containers of minimum size of memory (say 512MB or 1 GB). The ApplicationMaster can request any container as a multiple of the minimum memory size.

Source, see section ResourceManager/Resource Model.