Open in lets you choose an image-editing application which will then open the selected picture.

Reveal in Finder opens the folder that contains the image file. You can then work with all the files in the folder; they're still displayed by PhotoGrid.

Rotate allows for an upright view even of images you might have taken in a vertical orientation in a camera. Note that the images themselves are unchanged; only the PhotoGrid display changes.

Move/Copy ... is one of the most useful features of PhotoGrid.

Move into Trash deletes the image file from the folder it's in and moves it to the trash. It's the only command in PhotoGrid that alters the contents of the image file folder.

Remove from GridView deletes the image from the grid, but leaves the original file untouched. When you open this folder again in PhotoGrid, the image reappears.

Reload is only for use in rare cases when an image doesn't load properly into PhotoGrid.