A UK ETA and a UK visa are both related to permission to travel to the United Kingdom, but they are used for different purposes.
A UK ETA is not a visa. It is an electronic travel authorization for certain travelers who can visit the UK without a visa but still need permission before travel.
It is mainly used for short visits such as tourism, short business trips, visiting family, or transit.
A UK visa may be required for purposes that are not covered by a UK ETA, such as work, long-term stay, long-term study, or settlement.
Before traveling to the UK, it is important to check your nationality, purpose of visit, and length of stay to decide whether you need a UK ETA or a UK visa.