Whats the difference between a 3G broadband USB Modem and a laptop internet card?

Answer 1

The difference is 'USB' V 'card'

... the USB item plugs into a USB socket (duhh)...
... since the 'card' is for a laptop, I expect it will be a PCMCIA type.

USB is standard on all new laptops, PCMCIA is not (it's an old standard that you would find on most older Laptops).

Also, '3G' is a modern Mobile phone standard whilst 'Internet card' could be almost anything.