Report in Salesforce

Its a list of records that's displayed based on the filter criteria that we define. 

It’s displayed in rows and columns, and can be filtered, grouped, or displayed in a chart.

Compulsorily, report has to be stored in a folder which can be public, hidden, or shared, and can be set to read-only or read/write as per requirement.

Additionally, we can control who has access to the contents of the folder based on roles, permissions, public groups, and license types etc.

Reference : 

https://trailhead.salesforce.com/content/learn/modules/reports_dashboards (Classic)

https://trailhead.salesforce.com/content/learn/modules/lex_implementation_reports_dashboards (Lightning)