Download the official Scrum Guide
Scrum is defined completely in the Scrum Guide by Ken Schwaber and Jeff Sutherland, the originators of Scrum.
Web Guide
http://scrumguides.org/scrum-guide.html
Download Page
http://scrumguides.org/download.html
Italian Language PDF (2013) by “Scrum Italia Group”
http://scrumguides.org/docs/scrumguide/v1/Scrum-Guide-ITA.pdf#zoom=100