Digital "soft" or monitor proofing:
• no hard copy; WYSOSISLWYG
• generally not for contract proofing (exceptions: newspapers; short-run printing)
• OK for remote, first-stage customer approvals

Digital "hard" proofing:
• despite limitations, gaining in popularity
• various methods used
• each is completely filmless
• each uses dyes or pigments, not ink
• most give continuous-tone output because they can't generate halftone dots as photomechanical proofs can
• most can't proof to actual printing stock or comply with SWOP guidelines
• most can't produce full-sized imposition proofs

Some digital proofing technologies:
• laser ablation: the laser "blasts" tiny spots of dye from a donor sheet to a receptor sheet (Kodak Approval)
• dye sublimation: CMYK dyes on a ribbon are vaporized with heat; the proofing stock absorbs (sublimates) the colored gas
• ink jet: continuous-flow (Iris Realist) and "drop on demand" (Encad Novajet Pro)
• phase change, a.k.a. thermal wax (Tektronix Phaser)
• IR thermal laminate: creates halftone dots on a thermally activated proofing material that can be exposed in a platesetter (Imation digital Matchprint) (same device for proof and plate: the "grail" of the digital workflow)