Whats difference between, title, caption and keyword?

To add to Keith's summary:

• Titles are having the advantage, that you can see them below the thumbnails, when browsing your photos on a Mac, see for example: How to view the titles or filenames below the thumbnails in Photos 5.0 - Apple Community
• Titles can be used as the filename, when exporting photos from Photos on a Mac.
• Titles are shown above the photo, when viewing photos enlarged.
• Titles are syncing with iCloud Photos between your devices, but you cannot see or modify them on an iPhone or iPad. They will however been shown as a search result on your iOS devices.
• Captions are shown on your iPhone, iPad, iPod touch and will be synced with iCloud Photos to your Mac.
• Keywords are only shown in Photos on the Macs, and can only be modified on the Macs, but are also synced to the iOS devices and are shown in the search results.
• Keywords are a powerful organisation tool, as we can use them in smart albums and in the filters and are quick and easy to apply with keyboard shortcuts. One drawback is, that keywords are omitted, when we import photos from one photos library into another library. I am using Power Photos to move photos between libraries, to save the keywords. Importing photos directly from another Photos Library into the currently open Library

If you want to sync annotations between devices and see them on all devices, add them to the captions. As I want to see short annotations on my Macs below the thumbnails, I am using the titles on the Mac, but the detailed information is going to the captions. The titles and keywords are useful on the iPhone and iPad as well, as I can use them in searches.

Add titles, captions, and more to photos using Photos on Mac - Apple Support

I think of it this way:

Title: Short and Pithy => "George and Mary in Front of Castle"

Caption: More Info => "Here are George and Mary in front of Cinderella's Castle as he is proposing"

Keyword: These are used to "tag" similar photos. For example, keywords for this image might include "Disney", "Marriage", "Vacation". My keywords include "Disney", "Balloons*", "Nature", "Museum", "Train", "Remodel" and so on.

Actually, I don't bother with Titles and Captions. Just Keywords.

*I am in Albuquerque. My avatar is actually an hot air balloon.