For a quick description of standard Dolibarr features, take a look at the module description pages.
Go to the module description links on home page.
More official Dolibarr documentation is available online in the Dolibarr wiki website. You will find users, developers and translators documentation but also information about Dolibarr project.
To go onto wiki web site, click https://wiki.dolibarr.org
You may also find non official documentation (nor written, nor validated by Dolibarr dev team), available as PDF files, onto Dolistore (the Dolibarr market place). Search on DoliStore.