can someone explain the difference between Photos, Iphoto, Pictures

Too bad that we are in different time zones. That makes having an orde ly discussion difficult. 😁

Ok, your screenshot looks like I expected.

Your Pictures folder contains fragments of an iPhoto Library, that is no longer working. That is why the system is treating Pictures like an iPhoto Library. And inside the Pictures folder are your old files, that you need to save. It is good that you could open the Pictures folder by ctrl-clicking.

I would suggest the following:

• Quit Photos, if it is running.
• Make a backup copy of the contents of Pictures on an external drive.
• Create a new folder "My Pictures" in your home folder, outside of Pictures. keep a Finder window of your new "My Pictures" open.
• Now ctrl-click your Pictures folder again, use Show Package Contents and drag all files you need to save to your new folder "My Pictures". Move all photos, pdf files, and folders with photos over to My Pictures, but most importantly, the following items:
  • All Libraries: Photos Library.photolibrary, Photo Booth Library, iPhoto Library.migratedlibrary, Masters, Previews
  • Look into iPhotoLibraryOld
  • Your folders Children etc.
  • 
• Once you moved all important items with photos and pdf documents to your new "MyPictures" folder, delete the remaining parts of the old iPhoto Library, "AlbumData.xml", "com.apple.iphoto.plist","Info.plist", "iPhotoAux.db", "porojectDBversion.plist", "Project.db".
• Now test, if the Finder will still treat the Pictures folder as a photo library. If not restart, the Mac.
• If the Pictures folder is still behaving like a Photos Library, make a backup of your Mac, open "Disk Utility" and use the command "File > Run First Aid". It will fix permission problems with your user home folder.
• If your Pictures folder is now back to normal, you can move the files from "My Pictures" back to "Pictures", or just use them from your new custom My Pictures folder.

Your Pictures folder seems to have changed its type. It is still showing the icon of the system Pictures folder, but the "Kind" changed to Photos Library. Perhaps you can change this back.

Select the Pictured folder in the Finder and enter the key combination ⌘I (short for Full > Get Info).

What does the Info panel show?

It should look like this, with Kind: Folder

Is the Name & Extension field writable, if you click the padlock in the Info panel? If yes, can you disable the "Hide Extension" field?

This screenshot looks promising.

If this "Pictures" with the camera icon on the folder is directly inside your home folder, try to use the "Open With" menu to change this from"Photos" to the "Finder". Do not click the "Change All" button.

The Finder.app is a little hard to find: /System/Library/CoreServices/Finder.app/

In the "Other" file chooser panel click your system drive, then open the "System" folder, then "Library", the "Core Services" and scroll down to Finder.app.

To be able to select the Finder, you may have to switch the the "Enable" selector at the bottom of the panel to "All Applications".

it is simple but when you have questions you have to actually share details and ask clean, clear questions - this article will help ask better questions which will help you get better answers - Writing an effective Apple Support Communities question

As to photo organization software this forum is only about Photos for Mac which is included with the OS on all Macs - Pictures is not software it is a finder folder which is the default location for the Photos library and the recommended place to store photos - iPhoto is an older discontinued program different from functionally similar to Photos, forget it as there is no reason to learn and use it now

as to

when I open "pictures," for example, it opens a window saying it's preparing the library

that is not correct - Pictures is a finder folder and when you open it you see all of the files and/or photos stored in it - no program opens at all

and when you do open a program (probably Photos in your case) and you get an error, to get help with the error you actually have to tell us what the exact error is adn exactly what you did - there is no possible way for anyone to help with

something or other is locked, etc etc.

Read through the Photos help topics and ask a real question with details about that you have and what exactly is going on - Welcome to Photos on Mac - Apple Support

And these may help too - Photos - Official Apple Support and http://help.apple.com/macOS/high-sierra/whats-new/?lang=en&cases=kIFAGHvXTAepRpt QXiXe5w,N0bfKOvERSiAp06mHOcgOQ&notificat…

LN

So the Photos app is working fine then?

and it appears that you have names a Photos library Pictures with is probably not a good idea and will be confusing

When you launch the Photos Application go to the Photos menu ==> preferences ==> general and tell us the path to the Photos library that is reported at the top of that window

LN

Ctrl-click the Pictures folder and use the command "Show Package Contents"

This should be showing you a Finder Window with folders inside this folder.

Your Photos Library should be inside this folder, as your screenshot of the Photos Preferences is showing.

Please post a screenshot of the package contents of Pictures.