A Server Group is a grouping of one or more server instances that are subsequently treated as one when pertaining to configurations and deployments.
A cluster is a grouping of two or more server instances that have its deployments load balanced and/or have high availability.
The difference between the two is that clustering does not necessarily depend on the server groups. For High Availability, it depends on who is listening on a particular multicast address. This means that one can potentially have multiple Server Groups that listen on a particular multicast port all be clustered together. For Load Balancing, it depends on either how your hardware load balancer or Apache and mod_cluster instances are configured.
Additional to Adil Arif you
a server-group is a domain administration thing. I would recommend to use a have one server-group for clustered servers unless you have a good reason (i.e. slightly different config or deployment).
Also it makes configuration complicated if you build such complex scenarios