Amorphous is Thin-Film. Avoid it at that price!
It looks like brown paint, with lighter stripes going one way.
Crystalline, mono or multi, looks like squares, or octagons, or semi-circles, or something. But you can see each piece is seperate, and can see the ribbon wire connecting them.
Mono and Multi. Not much real-world difference. Look at the brand and $/watt.
Multi often looks like blue cracked ice.
Mono often looks like tiny glitters on a blue or brownish piece.
Make sure they have at least 36 cells (squares, semi-circles, etc). 32 or 33 will not work out so great in a 12V system.
G-