The early rabbis instituted it to protect the wife from a husband divorcing her for any reason - even if she burnt the soup! Whilst it is not sacred, it is nonetheless a binding contract which must have certain details concerning the time and date and place of the marriage, reference to the dowry and should be signed by two valid witnesses.
It is usually read out in full at the wedding under the chupa (canopy). The orthodox text is written in Aramaic (see above) whilst a shorter more egalitarian Hebrew text is favoured by the Progressive movements.
Whilst there are masses of rules governing the other scribal documents, there is considerable freedom with k’tubot, as they may be written by anyone (male or female, on any permanent kind of material, parchment, vellum, fine-papers, fabric, glass, stone or wood and do not have to be hand written but can be printed. Moreover they have been a source of Jewish art through the ages as they are generally embellished with designs and motifs and a personalised hand-drawn k’tubah is a much prized possession.