Hacking A bit confused about IOS and firmware

[cable-one]

Hiya,

Iam a bit confused about the IOS and firmware thingy.

I tought that installing a custom IOS is actually changing the firmware of your WII.
So iam on version 3.3U wich is the most recent verison before the 3.4 update.

Now if i install a custom IOS ( like i did for the backup loader) iam basically running a custom firmware? (for instance 3.3U - custom?)

Please inform me if iam totally off here?