Duplex printing is when impressions are placed on both sides of a sheet of media. Due to a general lack of conventions, the orientation of each of the impressions varies from vendor to vendor, and has changed over the years.
duplex, lduplex
Print on both sides using the default orientation. The lduplex is an alias for duplex
duplexshort, sduplex
Print on both sides but reverse the orientation of one page. The sduplex is an alias for duplex. Which page is reversed is at the whim of the firmware implementors and conventions for the printer.
simplex
Print on a single side of a page
tumble, shortedge
This is use to print a single page on one side of the media, but using the (nonstandard) orientation for the duplexshort. This is usually done when a single impression must be generated on the alternative side of the media, rather than the default side. Again, this is dependent on the whims and whimsys of the printer firmware implementors, and may have some unexpected side effects.