Welcome
The E series is aimed at the beginner.
The S is an Arranger series which is the next rung up the ladder from an E series.
The EW series are similar to the E series but with 76 keys
The A series is another Arranger but lower cost and aimed more at home entertainment.
There is also the Y series which are first keyboards hence aimed at children
All in all there is about 15 different keyboards produced so deciding which Yamaha is right for a users needs is a nightmare.
As a beginners keyboard a Yamaha is a good first rung but not necessarily the best for the second keyboard. It all depends on the users needs.
Casio make some excellent alternatives.
At a $400 price point probably the one now to set the standard is a brand new model and its by one of the best manufacturers, Korg; its their EK50.
Good luck making sense of the options.