I'm almost skeptical of a device that third parties seem to love this much...

You've never had to use an SD card? Mobile phones use them all the time. Anyway:

http://www.memorybits.co.uk/memory_cards

It almost feels like a jinx, doesn't it?

So its the same as a USB card? I have used one of those once. Another name for them is flash memory?

Not quite, you can get a USB adapter that you can put the memory card into and then connect the USB part to your PC. Probably should have directed you better:

http://www.memorybits.co.uk/categories/sd-gaming/785

The Wii uses SD cards for memory too. Flash memory is essentially memory with no movable parts like a USB memory stick, SD card, or any other type of memory card.

So to get the photos from the 3DS to the PC I can use the included SD card but I need to also buy a USB adapter?

Buy them in one package, cost almost nothing:

If your computer has an SD Card slot you won't need anything else. And I doubt the 3DS will come with an SD Card. They're very cheap however.

Probably, unless you have a SD card slot on your PC/laptop. You can usually get both in a bundle-type deal so it shouldn't be too much of an extra expense.

It's even possible to even not know what an SD card is?

It's just as common as usb.