Arash, Work Performance Information is an output of Direct and Manage Project execution.
WPI mostly includes :
Status of deliverable's
Progress of schedule
Risks status ,new risks if identified any issues or risks that have become issues.
Costs information like costs incurred for deliverable's ,procurement cost and any other costs.
Information on procurement.
These are routinely generated and collected as the project progresses and as the team is doing the planned work. This information can be termed as just data generated as of date.
The WPI then becomes input to the most M&C processes viz. Control cost, Control Schedule,control scope and one executing process(Perform Quality assurance).
The M&C processes use the WPI and compare it with baselines in the project plan e.g.cost data is compared to the cost baseline, the schedule progress is compared against the schedule baseline. These measurements are expressed in terms of variances Cost variance(CV) ,Schedule Variance(SV) and performance index CPI and SPI which gives the idea and measure of project progress and cost utilization etc. to the PM and management.
WPM then becomes an input to the other M&C processes viz. Perform Quality Control and Report Performance which utilizes WPM to generate forecasts ETC ,EAC TCPI etc which form the core of Performance reports.
Please note that Performance reports are extensive and carry most of the information like variances, CPI,SPI and forecasts along with issue status and newly identified risks and status of old risks etc.
I hope this helps.
-Sagar ,PMP