Starting on Boot
A default install of Archlinux will leave you with very few services (or daemons) enabled during boot. You can add or remove services by editing the the daemon array line in your rc.conf file. It will initially look something like this:
DAEMONS=(syslog-ng network netfs crond)
They will start in the order you have them listed. You can disable one and keep it in the array by prefixing it with an exclamation mark (!). You can also have them start in the background by adding an at (@) symbol in front of it. More on that here.
Manual Starting and Stopping
You can see what service start up scripts you have by looking in your /etc/rc.d/ directory. You can also manually start, stop, and restart them by issuing
They may also have other commands, check with the documentation.
You don't necessarily have to add anymore if you don't feel the need. Typical desktop user may want to add Cups, HAL, and Alsa. Note, some services will start other services. For example HAL will automatically start DBUS and Acpid. Keep in mind as you install new services you will have to manually add them to rc.conf.
Starting Daemons in Background
This is helpful for starting a service and letting the next service start before the previous one has finished. Which services to start background depends on your needs. Don't background syslog-ng, network, netfs, hal or anything you need immediately.