ET comes from the German "Einpresstiefe". It's the distance, in millimetres, from the centre line of the wheel to the face that bolts against your hub.
Positive ET puts that face towards the outside of the wheel, so the wheel tucks further in under the arch.
Zero ET puts the face exactly on the centre line.
Negative ET puts the face towards the inside, so the wheel sits further out.
The lower the number, the further out the wheel sits. That's why low and negative offsets look aggressive.
Width works alongside offset. A wider wheel at the same ET sticks out further on both sides, towards the arch and towards the brakes.