Probably the best choice of holiday for singles. There is usually quite a number of people travelling on their own. People seem to mingle easily so you will not feel left out. Cost can be a bit of a problem as, like all holidays, cruises are made for couples. None the less, there are quite a few cruise companies who have ships with single cabins eg Fred. Olsen, SAGA, P&O, Cunard. You can also book a double cabin for single occupancy but you will have to pay a premium which can vary from 10% upwards.
I have travelled alone on quite number of occasions and have found it an most enjoyable experience. I can recommend it.
P&O dont have single cabins, they are Double cabins with Single occupancy supplements, now from 45% to 80%.
A properly phrased question would be helpful! What exactly would you like to know, or would you just like us to guess?