It is not a visa
The ETA is a pre-authorisation for travellers who do not need a visa for the UK. The check happens before boarding: without an ETA, the airline can refuse to carry you.
Unlike a visa, there is no appointment and no interview. Everything is done online.
Linked to your passport
The authorisation belongs to the passport you applied with. If you get a new passport, you must apply again — even if there is time left on the 2 years.
There is nothing to print: at the border, the same passport is enough.
Who needs one
Travellers from visa-free countries, which includes Australia and New Zealand, whether for tourism, visiting family, business or transit.
Children and infants each need their own ETA.