The command is simply
open (which can also be used for opening directories). The most basic example of launching an application:
open /path/to/some.app
More complex possibilities also exist:
open "/Volumes/Macintosh HD/foo.txt"
opens the document in the default application for its type (as determined by LaunchServices).
open /Applications/
opens that directory in the Finder.
open -a /Applications/TextEdit.app "/Volumes/Macintosh HD/foo.txt"
opens the document in the application specified (in this case, TextEdit).
open -e "/Volumes/Macintosh HD/foo.txt"
opens the document in TextEdit (the
-e option specifies TextEdit).
open http://www.apple.com/
opens the URL in the default browser (lynx, naturally *wink*)
open "file://localhost/Volumes/Macintosh HD/foo.txt"
opens the document in the default application for its type (as determined by LaunchServices).
open "file://localhost/Volumes/Macintosh HD/Applications/"
opens that directory in the Finder.