I heard that apt-get upgrade command used to upgrade the packages up to date,
but when I tried to use it, there was no upgrade for me. my commands were

apt-get update
apt-get upgrade
then system show
Reading Package Lists... Done
Building Dependency Tree... Done
The following packages have been kept back:
arts bind9-host dnsutils kdeedu klettres kstars libarts1 libnspr4 libnss3
mozilla mozilla-browser mozilla-mailnews mozilla-psm
0 upgraded, 0 newly installed, 0 to remove and 13 not upgraded.

Why the 13 packages were not upgraded?

When you get that do a apt-get dist-upgrade

I have not tried apt-get dist-upgrade before, should I do?

when I tried it, most of the packages were upgraded and none of no upgrade package found but they tried to remove my mozilla - related package as show below:

Reading Package Lists... Done
Building Dependency Tree... Done
Calculating Upgrade... Done
The following packages will be REMOVED:
mozilla mozilla-browser mozilla-mailnews mozilla-psm mozilla-xft
The following NEW packages will be installed:
jackd kbruch kig klettres-data kstars-data libboost-python1.31.0 libdns16
libjack0.80.0-0 libsndfile1
The following packages will be upgraded:
arts bind9-host dnsutils kdeedu klettres kstars libarts1 libnspr4 libnss3
9 upgraded, 9 newly installed, 5 to remove and 0 not upgraded.
Need to get 17.7MB of archives.
After unpacking 20.8MB disk space will be freed.
Do you want to continue? [Y/n]

...sometimes, with SUSE 9.1 (via APT), I've had to remove packages and then reinstall them afterwards. Why it works this way (with SUSE/APT at least), I don't know.