Analytic Snapshot:

Analytic Snapshot:
Reports display data at a given time.Analytic snapshots capture and store data at scheduled intervals(e.g.,every Friday).Steps considered shown below

Once you created source report and target object with custom fields of similar data types on report fields then create analytic snap shot from setup-->administration setup-->data management-->analytic snapshot-->new enter the details and map the fields like this

Finally schedule snapshot by scheduled analytic snapshot option.
Build a report based on the historical data using target object report and  analyze historical trends each week like this.