The National Visa Center (NVC) assigns each immigrant petition a case number. The NVC case number has three letters followed by ten numbers. The three letters are an abbreviation for the overseas embassy or consulate that will process the immigrant visa case (for example, JAK for Jakarta, MNL for Manila).
The digits tell the NVC exactly when it created the case. For example a case with the number MNL2011747003 would be a case assigned to the U.S. Embassy in Manila. 2011 is the year in which NVC received the case from the USCIS. The Julian date is 747 plus 500, so this case was created on September 4, 2011, the 247th day of the year. The 003 shows that it was the third case created for Manila on that day.
The NVC case number is not the same as the USCIS receipt number, which is written on the Notice of Action, Form I-797, from the USCIS. A consular section abroad cannot find a case if all you have is the USCIS receipt number.
Go here for a definition of the USCIS Case Number.