Three groups of travellers
Anyone travelling to the UK falls into one of three groups: British/Irish citizens (no ETA needed), visa nationals (need a full visa, not an ETA), and eligible non-visa nationals (need an ETA).
How to check
The quickest way to check is to look up your nationality in the eligible list before applying, thee list gets updates frequently.
Common mistakes
Assuming a previous UK visa means you're exempt
Applying for an ETA when you actually need a full visa
Leaving it until the last minute