From the Nmap book:
By default, Nmap does host discovery and then performs a port scan against each host it determines is online.
Nmap works in two stages: a "probe" stage (sometimes confusingly called a "ping" stage) and a "scan" stage. These two stages roughly correspond to the -P and -s classes of options.
The idea is that you want to scan some (probably big) list of TCP and/or UDP ports on machines in a set of ip addresses (a large subnet, say) but you don't know in advance which ip addresses in that subnet are even up and running. Since sending a bunch of scans to an ip address that is connected to nothing would take a looong time, nmap sends a small number of packets of specific types during the probe stage to every ip you specified. Only ip addresses from which a reply is received are then scanned in the scan stage.
Confusingly, there are some -s options that actually specify things from the probe stage. I think the one you're thinking of is -sn ("scan none" is what I remember), which means "skip the scan stage entirely and just report which hosts are up". As the nmap book says:
This option tells Nmap not to do a port scan after host discovery, and only print out the available hosts that responded to the host discovery probes. This is often known as a “ping scan”... In previous releases of Nmap, -sn was known as -sP.
I presume this is what you mean by "ping scan", since the nmap book refers to it this way, and the old -sP option does lend itself to the "scan Ping" mnemonic. Now "ping" is the actual name of a common command line program used to see if a given computer is on the network. The ping program works by sending an ICMP Echo packet to the host in question, and sees if the host in question responds. There are no "ports" to send an ICMP Echo request to, because the ICMP protocol, unlike it's siblings TCP and UDP, does not have the notion of ports, only different types of requests.
ICMP specifies several types of requests that nmap can use to determine if a host is up. -PE sends an actual ICMP Echo request, which is what the ping command sends. It can also send ICMP Timestamp (-PP) and ICMP Address Mask (-PM) packets during the probe stage.
