Hi Gill,
welcome to the Apple (user-to-user) Community.
Re: "What’s the difference between folders and albums?"
This is the way I understand it:
Folders hold files created / saved / in a great many apps.
You can generally navigate to them easily to find the file you want
In the Photos app: Folders are called Albums.
You need to first go to the Photos app, in order to access those files.
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Re: "Is there a way to clearly see if you’ve missed sorting certain photos into their appropriate album that you’ve created."
One idea: Stay in the Library view, and just work through the Photos (left to right & row by row) dragging them to the appropriate Album(s) one by one, or in groups when they are side by side. (Keep track of where you are up to if you take a break). Then, when you are done with the Library, go view / organise the album
Else:
Another way of working may be: In the Library, carefully select everything you want in a particular album, (eg: if you want: note the filenames (IMG-[number] tho it isn't necessary) and, only when you are sure you have selected everything, drag them all to the album you created in one go.
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Apple support have a useful article online:
How to: Organize and find your photos on your iPhone, iPad, or iPod touch - Apple Support
Eg: says:
Add to an existing album
- Open Photos.
- Tap Select, then tap the photos and videos that you want to add.
- Tap the Share button , then tap Add to Album and choose an album.
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If you use iCloud.com
(Although it doesn't mention Photos in Albums)
How to:
Organise files and folders in iCloud Drive on iCloud.com – Apple Support
All the best :-)