Backlog is the stack of user stories that describes the features or technical task or non- functional requirements that needs to be delivered…
There are no strict requirements as all the user stories need to be very detailed, however the features or technical task that need to delivered soon should be having more details or more granular rather than the one that is lower in priority…
The product backlog is the stack of EPIC and User Stories under EPIC in the order of their priority. The high priority user story remain on the top and picked up for current iteration, while the user stories that are lower in priority in terms of business value remains lower in stack.
The stack of user stories that are picked up for current iteration forms the sprint backlog.