The difference is the active choice in priming the action
A readied action is an action that a player would choose to hold onto until a trigger occurs, at which point the action is performed. They have to have the forethought to expect some circumstance that they wish to react to.
A triggered-action doesn't require that kind of forethought. It's an action that is effectively "always on stand-by" without you needing to do anything.
Take for example, attacks of opportunity
For many games, an attack of opportunity would be a kind of triggered action because you don't have to explicitly have to be on the look-out for a situation to arise to take one. An enemy moves out of your reach, or casts a spell in front of you, or drinks a potion while engaged with you and it triggers a response.
That could be the tag guidance for triggered-actions
Triggered actions are actions that you take in response to some stimulus without having to explicitly anticipate that circumstance beforehand.