The Export action can export four types of image from Aperture:

Export Version - this is the regular export using an existing image export preset.

Export Master - export the Master files, with the option of adding XMP sidecar files or embedding metadata. Note that exporting Masters is disabled for Aperture 1.5.6 due to an AppleScript bug.

Export Preview - this exports the Preview for the Version, assuming there is one present.
This allows for very fast exporting of files (just a file copy) if Previews are generated, but is dependant on the settings when the Previews were generated. Note that Preview files are in Adobe98 colour space, and only contain the metadata that had been added when they were generated.

Export Thumbnail - this is the 1024px thumbnail that is used internally by Aperture for laying out Books, Web Pages and Light Tables and for low quality slideshows, and for Quick Preview mode if no Previews are present.
Exporting Thumbnails is slightly slower than exporting Previews as the file has to be extracted from the AP.Thumbnails file inside each Project, but it's still fast and can be done for any image visible in Aperture, even if the Master is offline or lost and no Preview was generated.
Thumbnails are in Adobe98 colour space and contain no metadata.