Web Application vs Site Collection

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Hi,

I am trying to find some information about the things to keep in mind when
starting a SharePoint project.
In particular:
* When to decide to create separate Web applications?
* When to decide to create separate site collections?
* When to decide to create separate sites?

On which criteria should one decide to create a separate such part?

I did understand already some criteria to determine a choice:
* Backups can only be taken per site collection seperately.
* The database storing the data can only be set seperately per site
collection not per site.

I am looking for other such criteria, especially also related to security.

Is there good documentation somewhere about this topic? Because I don't find
anything explaining clearly the differences between web applications-site
collections-sites.

Thxs