'localhost.localdomain' is the fully qualified domain name of your local PC as it references itself, and 'localhost' is an alias that is equivalent to 'localhost.localdomain', the only difference being that it is shorter. No matter what hostname is set for your PC, the names 'localhost' and 'localhost.localdomain' will always work as a reference to itself. Both will always point to IP 127.0.0.1 (unless you explicitly temper with the /etc/hosts file), which is your current PC.
If you don't know what domain name to use instead of localhost, than you have no domain name, otherwise you would know about it. The two possibilities are:
1) use any name you want if this is your home PC or a PC on a local network with no server services to the outside world
2) register some domain name (your ISP is likely to have this service) if you want your server to be seen from the Internet identified by that name
If you have no specific reason for a unique domain name, than leave localhost as it is. Otherwise specify what you want to achieve.
Hope this helps,