apt and yum are a way to install new packages and update existing packages. One advantage is that dependancies are resolved when installing.
You can run the following to update all the packages: 'yum update'
You can install packages with the following: 'yum install <package name>' ie: yum install gkrellm
APT is very similar except the update is 2 steps 'apt-get update' and then 'apt-get upgrade'
ON fedora I don't know if APT is installed by default but I think YUM is.