The container format refers to whether the virtual machine image is in a file format that also contains metadata about the actual virtual machine.
There are two main types of container formats: OVF and Amazon's AMI. In addition, a virtual machine image may have no container format at all --basically, it's just a blob of unstructured data...
You can set your image's container format to one of the following:
ovf
This is the OVF container format
bare
This indicates there is no container or metadata envelope for the image
aki
This indicates what is stored in Glance is an Amazon kernel image
ari
This indicates what is stored in Glance is an Amazon ramdisk image
ami
This indicates what is stored in Glance is an Amazon machine image
