*.PDB - The .PDB extension stands for "program database." It holds the new format for storing debugging information that was introduced in Visual C++ version 1.0. In the future, the .PDB file will also hold other project state information.
While earlier, 16-bit versions of Visual C++ used .PDB files, the debugging information stored in them was appended to the end of the .EXE or .DLL file by the linker. In the versions of Visual C++ mentioned above, both the linker and the integrated debugger were modified to allow .PDB files to be used directly during the debugging process, thereby eliminating substantial amounts of work for the linker and also bypassing the cumbersome CVPACK limit of 64K types.
(From MSDN)