In the field of graphic design, a proof is a reasonable facsimile of final product appearance presented to obtain final approval. At the very least, it may be a B+W photocopy sent via fax. A step up might be a color copy printed with a different method than the final. So a color laser copy might simulate how a brochure to be printed offset will appear. For offset printing direct-to-plate, digital inkjet proofs calibrated to typical press conditions (contract proofs) are the best option.
To minimize risks associated with printing color-critical large posters, for a fee, a small sample inkjet printed poster may be requested using the actual paper and inks. When the only question is how particular Pantone colors will reproduce, a swatch test print on the desired paper can be requested for a nominal fee.
Each of the proofs mentioned thus far are considered hard proofs since there is a physical object to hold and view under various lighting conditions. In some cases, approvals are made based on what is known as a soft proof viewed on a computer display. The common file format for soft proofs is a PDF. Adobe Acrobat is typically used for viewing with a color calibrated monitor. Without proper calibration, colors are only general approximations. Even with calibration, because computer displays emit light, colors and dark shadow detail can appear brighter on a monitor than they do after printing on paper.
For all camera-ready art submitted for offset printing, a soft proof should be approved prior to placing the job on the press. Alterations after viewing a proof at this final stage are billable.