But assuming it's an OCI container, which will be the case if you're using any common managed container runtime and not rolling your own, it's a process in its own user namespace, mount namespace, UTS namespace, cgroup namespace, IPC namespace, and time namespace. It's assigned its own hostname and IP in the new UTS namespace. It runs in a chroot in the new mount namespace to a root filesystem assembled via overlay with a single writeable layer on top of N read-only layers shared with any other container that is launched from the same image. It gets bind mounts, kernel VFS mounts, an environment, and an entrypoint command from a config file colocated with the root filesystem in a container bundle created from the image defaults, system defaults, and command line overrides.
The simplest way to compare it to a VM is a container is the Linux kernel using namespaces and cgroups to scope the services and resources it presents to a single userspace process. A VM is a hypervisor presenting virtualized hardware and BIOS services to guest kernels.