Installing software globally

Probably the most common case, is wanting to install software system-wide. This is not  technically system-wide in NixOS, due to its internal isolation properties, but the difference doesn't really matter for your day-to-day use. 
 Installing software globally is done by adding it to your environment.systemPackages . For example, like so: 
 {
 # ... other configuration goes here ...
 
 environment.systemPackages = [
 pkgs.htop
 pkgs.iotop
 ];

 # ... other configuration goes here ...
} 
 In this example, two packages are installed: htop , and iotop . As you can see, they are prefixed with pkgs. - this means that they are attributes (ie. properties) of the pkgs binding (ie. variable).  The type of environment.systemPackages is a list , containing derivations . 
 Once you have added the package(s) that you want to install to your systemPackages , you need to rebuild the system to make the changes take effect.