UK ETA
Designed for short visits (tourism, business, transit) by eligible non-visa nationals. Lightweight online process, no interview required.
Standard Visitor Visa
Required for visa nationals, or for longer/more complex visit purposes. Involves a fuller application and sometimes biometrics.
Which one do I need?
If your nationality is on the eligible list and your trip is a short visit, the ETA is almost always the right (and faster) route.