By default WordPress processes all uploaded images and creates 3 sizes: Thumbnail, Medium and Large. You can set their dimensions in Media settings of your WordPress dashboard. Starting from version 2.9 WordPress added the possibility to add other sizes besides that three. However, there is no interface for this. These additional sizes are set in your theme functions.php file.
“Post-thumbnails name” dropdown contains all sizes, available in your WordPress, except Large, as I thought there would be no reason to show Large thumbnails.
So, you have “thumb” and “medium” default sizes, and somewhere in your theme you also have the declared “post-thumb” size. You can add your own size with any name there with desired dimensions. This is very nice feature.
You can read more about this feature and how to add and modify sizes in your theme functions.php file here: