from where to get the package ?
Doesn't matter. Yast or zypper will download the package
from a "Suse server", and install the software automatically.
what extension file is it ?
Suse packages are .rpm packages.
No extension is used when you search (zypper se) or install (zypper in).
Just try with some small basic packages. Example :
# zypper se Terminal
# zypper in Terminal
The most common 14,000 applications / libraries are available from Suse.
Additional software http://en.opensuse.org/Additional_package_repositories
There will almost never be any need to download a package manually.
So you don't have to care about package extensions.