There are different types of proofs available.
Lasers are quick and inexpensive to produce. Confirms file contents, shows position, color splits, and copy so you can check for any necessary type corrections. Fattens the appearance of thin type and photo reproduction quality is mediocre.
ris Proofs are used to show content and folding. Iris proofs can be 1/s or 2/s. They show color split but are not used for accurate color.
Epson Color Proofs are pretty accurate. Shows 90-95% color accuracy. Color images look brighter and sharper than they will on press. Black and white photos look somewhat flat. The Epson (unlike the actual printing process) produces images without dots.
Press Proofs are produced directly from the printing press. Very expensive, since the press must be set up and operated to produce the proof, but sometimes necessary. This is the only true color representation of the finished product.
What to look for on a proof:
Are there typographical errors in the headlines or callouts?
Are all the pages there and in the correct sequence?
Do headers, footers, and folios line up?
Do borders and rules that crossover pages align properly?
Are all elements in correct register?
Have live image areas been masked over or trimmed out?
Are photos correctly placed, scaled, and cropped?
Have any images been flopped?
Is the trim size correct?
Is the folding correct?
Is each element the right color? Is the color vivid and rich?
Have critical color areas been accurately matched?
Are there blemishes, spots, or broken letters?
Were corrections from previous proof, if any, made correctly?
Check for typos again.