I'm wondering if there's a difference between iPhoto the application that came with my (then) new computer, and iPhoto APP, from the app store? I never could fine "iPhoto.app" on my computer.
If you are seeing "iPhoto.app" or "iPhoto" in your Applications folder will depend on your Finder Preferences, not on the application you installed.
If you disable filename extensions to show, you will only see "iPhoto" and not "iPhoto.app".
See this Help page: OS X Yosemite: Show or hide filename extensions
Filename extensions—the period followed by a few letters or words that you see at the end of some filenames (for example, .jpg)—indicate which app can open a file.
Filename extensions are usually hidden in OS X, but if you find them useful, you can show them. If extensions are hidden, OS X still opens files with the proper apps.
- Select a file, then choose File > Get Info, or press Command (⌘)–I.
- Click the triangle next to Name & Extension to expand the section.
- To show or hide the filename extension, select or deselect “Hide extension.”
If you select “Show all filename extensions,” all extensions are shown, even for files that have “Hide extension” selected. If you deselect “Show all filename extensions,” then file extensions are shown or hidden based on their individual “Hide extension” settings.
- Choose Finder > Preferences, then click Advanced.
- Select or deselect “Show all filename extensions.”
When you rename a file or folder, don’t change its filename extension. Or, you may no longer be able to open the file with the app that was used to create it. To be warned before you change an extension, select “Show warning before changing an extension” in the Advanced pane of Finder preferences.