My understanding is that Games Development is the general term for making games.
Inside Games Development there are several specialist disciplines. Roughly you could divide it into Code(Engineering ), Art, Design, Audio, Data Science, QA and Production.
A Games Designer is a certain type of Games Developer. Less involved with the visual look of the game ( this would be more the Artist’s area ) but more in charge of how the game might actually work.
Some design roles include Gameplay prototyping and Design, Systems Design, Level Design, Narrative Design ( often working alongside writers to create the story) economy and difficultly balancing.
Henry recommends the following next steps:
- Look up some of these specialist areas on Wikipedia
- Read about the MDA Framework