Definition of Pre-flighting

Pre-flighting makes sure that all files needed for the printing job have been correctly delivered to the printer.

Common problems detected in digital files provided to the printer by clients are:
Linked files are missing
Fonts used in the document are missing or damaged
Fonts are not compatible with the system used by the printer
Images used in the document are missing or damaged
Image files have a different format than required
Image files are in a different color spectrum
Absent color profiles
The document size is incorrect
The margins of the document size are too close or non-existant
Bleeds have not been accounted for
Document hasn't been prepared for color separations
Hidden elements cause an error when printing
Elements outside the printable area cause an error when printing
Layers have not been flattened

Does it sound complicated?
It could be, if you don't follow some guidelines nad have some know-how of what you're doing.
Most printers nowadays have their own pre-press department that handles the pre-processing of the digital files sent by clients. Many jobs get stuck in that previous check, due to any or many of the problems listed above.

For people that lacks the technical know-how to make sure the files being sent are properly prepared there are some software applications that will help them pre-flight the files.

Some pre-flight applications:

Markzware FlightCheck:
A standalone application that scans, verifies and collects each job for output.

PDF/X-1 Verifier 2.0:
This application verifies the integrity of PDF/X-1a files

Apago PDF/X-1 CheckUp:
Offers PDF/X-1 CheckUp 2.5, a plug-in to Adobe Acrobat that preflights and produces PDF/X files.

Enfocus PitStop Professional:
Preflights and edits PDF documents.

Extensis Preflight Pro:
Preflights and collects the job for output.

TIFF/IT-P1 Checker:
Targets TIFF/IT-P1 files.

Asura & Solvero:
Targets PDF, EPS and PostScript files.

Opens and view production-image format files, including TIFF/IT, CT, HC and LW.

Preflight Online:
A Web-based solution for printers and publishers.

Enables users to view the complete data in TIFF/IT-P1 files

SpeedFlow Check:
Preflights, edits and imposes PDF, EPS and PostScript files.

These applications work with most current DTP software such as:

Adobe InDesign
Corel Ventura
Impression Publisher
Microsoft Publisher
OvationPro - RISC OS
Publishing Partner

Other Graphic Design software software widely used to produce documents are:

Corel Draw