What does work means in Scrum

Words like "story" and "task" do not appear in the Scrum Guide. Instead, the Scrum Guide talks about Product Backlog Items, which may be decomposed by the team into smaller and more precise items. It is up to the team to decide how to structure Product Backlog Items (stories are one possibility) and how the Product Backlog Items are decomposed (specific technical tasks are one possibility).

I would describe the Product Backlog Item as a unit of value. It could also be a unit of work, but that is up to the team.

what does work means in Scrum?
Using Stories and tasks(the how of the story), is work related to the task or to the story?

Think of work as anything of value which requires the Developers to ensure it is Done. Tasks and stories are just ways in which work might possibly be represented and estimated.

From the Scrum Guide's section that explains the Product Backlog

The Product Backlog is an emergent, ordered list of what is needed to improve the product. It is the single source of work undertaken by the Scrum Team.

From that I say that work is anything the Scrum Team (not just Developers) need to do in order to improve the product.  User Stories, Tasks, Subtasks, Defects are all ways to describe a situation that will require work to improve the product.  There is work associated to creating all of those items which is done by various members of the Scrum Team.  Those items will describe situations, requests, problems that improve the product and will require some manner of work to satisfy the statements held within them. 

Using Stories and tasks(the how of the story), is work related to the task or to the story?

If your team has chosen to use User Stories and tasks, work could be related to either one of them.  But since User Stories and tasks are not mentioned or suggested by the Scrum Guide, there is no "Scrum" answer for your question.  If you want an answer, you should ask the Scrum Team by whom the User Stories and tasks are created. 

Thanks a lot!

I know that Stories are not part of Scrum. We had a discussion with some colleagues, mostly developers, regarding the "work" mentioned in the Scrum Guide. 

Ok, now it's clear.

Thanks a lot.