@noodlebin You would do well to click Docs at the top of the screen and step through the Wiki pages. Most of your lingo will be described on those pages in great detail. It's asking a lot of this forum for everyone to just shout out definitions of things. Best do a little research on your own and as specific questions.
As for overlays, this usually refers to images that are used in conjunction with an emulator that technically render over the game video, but are usually built with a big transparent "hole" in the middle and are sized so that they only fill in the pillar-box sides of games that are running on a widescreen. For example, an arcade game which was built around a 4:3 aspect ratio CRT won't fill a 16:9 TV screen, so an overlay can be configured for that game to display the artwork that surrounded the screen of an original arcade cabinet. There are other overlays that serve to apply semi-transparent effects such as simulated scanlines to give that retro CRT look to modern LCD displays, but most of us use a shader for that now.
Launch Images--This probably refers to an image that displays on the screen briefly while a game is launched. In its simplest form, it could be an image that just says "Loading your game" but it can easily expand to custom images per title as a way to add extra artwork to enhance the player experience. It could also refer to the SD Card image downloaded and written to an SD Card used to launch RetroPie on a Raspberry Pi.
