SharePoint Online GUIDs

SharePoint Web GUID
https://SPO/teamss/MySite/_api/web
In Each URL you can see..
<d:Id mLtype="Edm.Guid"XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX></d:Id>
The current site collection:
https://SPO/teamss/MySite/_api/Site
Web and Site Features and their GUIDs:
https://SPO/teamss/MySite/_api/web/Features
https://SPO/teamss/MySite/_api/site/Features

Lists:
https://SPO/teamss/MySite/_api/web/Lists
A single list (by name):
https://SPO/teamss/MySite/_api/web/Lists/getbytitle('MyListName')
A single list (by GUID):
https://SPO/teamss/MySite/_api/web/Lists(guid'h654d592-8574-3vr4-c231-40ad82e40a21')
All list items:
https://SPO/teamss/MySite/_api/web/Lists/getbytitle('MyListName')/items
A single list item:
https://SPO/teamss/MySite/_api/web/Lists/getbytitle('MyListName')/items(1) (Item with the ID of 1)
List items by criteria:
https://SPO/teamss/MySite/_api/web/Lists/getbytitle('MyListName')/items?$filter=Group eq 'IT'

All list columns, including both display name and internal name!
https://SPO/teamss/MySite/_api/web/Lists/getbytitle('MyListName')/Fields
In Each URL you can see..
<d:Id mLtype="Edm.Guid"XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX></d:Id>
Especially note these two elements:
<d:InternalName>PercentComplete</d:InternalName>
<d:Title>% Complete</d:Title>
The current user:
https://SPO/teamss/MySite/_api/web/CurrentUser