Basic Terms about Photos

As with any technical tool, the Photos app uses words that can have special meaning--maybe different from other uses.

The Photos app

contains the instructions for dealing with a database that contains information about your photo images. On a Mac, the Photos app can be found in the Applications folder, and probably in the Dock of apps at the  bottom (or, in my case, on the side) of your Mac. The Photos app isn't a huge file, because it doesn't have any pictures, just instructions.

The Photos Library

is a package (a slightly protected folder that has stuff you're not supposed to mess with) that contains the database and the Original picture files. It also has stuff like Thumbnails (the tiny pictures in the grid view of Photos) and Previews (screen sized images for showing quickly.) On a Mac, the Photos Library is kept (usually) in the user's Pictures folder. The Library is big, many gigabytes, because it contains all the pictures. On a Mac, you can have several Libraries and switch between them, but only the one called the System Library can be connected to iCloud. On an iPhone or iPad there can be only one Photos Library, and you can't see it or access it directly. 

The System Library

is the Library that apps, like Messages, or Screensaver, or Safari, go to when they use a picture or save a picture in Photos. It is the only Library on a Mac that can synchronize with iCloud Photos. On an iPhone or iPad, its System Library is the only one Library it can have.

The database is where all the information is stored for the pictures. As you add keywords or captions or titles, for instance, those aren't attached directly to the picture file, but they are stored in the database. This metadata is attached to the file (if you so instruct) when the file is exported for use outside of Photos. In fact, the Original picture file is never altered-- it remains available exactly as it was loaded no matter how you may make changes. The changes are recorded in the database, and they are applied when you view the image. Previews are smaller, temporary screen sized images that show the edits, but full sized edited images are created on the fly from the editing changes that were recorded. Even the original filename is no longer attached to the image-- a unique name is created for the file, and the original filename is stored in the database.

In Photos, the Library View

shows all the pictures in your Library. (That's why it's called Library!) Deleting a picture from there removes it from everywhere. You don't want a picture to disappear from the Library view, because then you wouldn't be able to put it in an additional album. The Library view can be sorted

by Date Captured or Date Added to the Library. Filters can be set to show only a particular type of picture.

Albums

are the primary organizing method in Photos. The thing is, in Photos, pictures aren't exactly "in" an album. When you "put a picture into an album," its name is added to a list of pictures from the Library that display together when the album is clicked, kind of like a music playlist, but for pictures. So two albums can both have the same picture name in their lists (like two playlists with the same song,)  and that picture will show up when you click either album, but there's only one picture file-- it's just on multiple lists. And when you remove a picture from an album, its name remains in the lists of other albums, and the file still remains in your Library. 

So albums give a specific view of your pictures. The picture of "Aunt Ethel at the Grand Canyon" can be in the "Aunt Ethel" album with other pictures of Aunt Ethel, and it can also be in the "Grand Canyon"  album with pictures of other people at the Grand Canyon. And it can also be in a the "September 2015" album with other things that happened then. Each album is pointing to a single file stored in the Photos Library, so having pictures in multiple albums takes up no more storage space. As you can imagine, this is very powerful in organizing pictures.

Albums can be in Folders

in the same way. Folders can "contain" Albums (again meaning having a list of album names and only looking like a container.) And Folders can even "contain" other Folders inside. But Folders can't have individual loose pictures.

Here is an article on using these features to organize pictures in Mac Photos:

Organizing Photos with Folders & Albums - Apple Community