Security groups define a set of IP filter rules that determine how network traffic flows to and from an instance. Users can add additional rules to an existing security group to further define the access options for an instance. To create additional rules, go to the Compute | Access & Security view, then find the security group and click Manage Rules.

Security groups are project-specific and cannot be shared across projects.

If a security group is not associated with an instance before it is launched, then you will have very limited access to the instance after it is deployed. You will only be able to access the instance from a VNC console.