New CentOS users will be surprised that so many popular packages doesn’t exists in default yum repositories.
For example, htop package not included in standard archive, it is also hard to find suitable rpm for your release.
In order to find lots of extra packages, you need to enable epel repository first.
The term EPEL comes from Extra Packages for Enterprise Linux. It can be enabled like that:
$ sudo yum -y install epel-release
After that, you can install lots of extra packages with yum, like htop.