In a nutshell a package is a complete program including configuration files, man pages etc. For example the command:
yum install evince
Will install the package evince (PDF reader) including dependencies if missing. A group are several packages bundled together. The desktop is an example of a group.
http://www.tecmint.com/20-linux-yum-...age-mangement/
There isn't a simple way to install all packages in a repository using yum. You would need to create your own repo and group everything together. Something like the following but will not install dependencies if outside the repository.
yum --disablerepo=\* --enablerepo=repo_name install \*