As said it's a very broad term, for example in the user a car detail is basically what we would call a valet.
I would personally say detailing a car is just taking your basic clean to the next level. So for example where most would simply think a sponge and chamois gets a car clean, a detail entails going that bit further. Claying your paint to prep it for polish, applying a glaze, sealant, wax etc. Obviously you don't take every step everytime you wash your car. You'd then consider possibly correcting paint as well.
Interior wise most people would consider a quick vacuum and wipe down ok where as a 'detailer' would consider cleaning and protecting interior plastics rather than just cleaning them. Same goes for window rubbers, engine bay parts etc.
There's no clear definition but a 'detail' of a car is that extra attention to detail you don't see on a valet.