difference img \ iso

[Osinnyy Bogdan]

Hi there.

I just want to try some freebsd and get stunned by choosing what release I
should to download\install.

So I ask you: what difference between iso\img, I mean, If I download
dvd1.iso and mount it on USB drive => starting installation, what difference
between this method and by using *.img ? Why am I asking, 'cause dvd1's got
3 times more weight, but I never faced with .img when installing windows
systems.

Thanks.