Polycystic ovaries contain a large number of harmless follicles that are up to 8mm (approximately 0.3in) in size.
The follicles are underdeveloped sacs in which eggs develop. In PCOS, these sacs are often unable to release an egg, which means ovulation does not take place.
It's difficult to know exactly how many women have PCOS, but it's thought to be very common, affecting about 1 in every 10 women in the UK.
More than half of these women do not have any symptoms.

Source : UK NHS

The syndrome is known to be associated with abnormal androgenic hormones and insulin, but the cause (which may be a complex inter-relationship between various factors) has not been accurately determined.

It is known to run in families, so there is clearly a genetic factor, but the gene(s) responsible have not been identified,

It's also known to be strongly associated with obesity - but it's unclear whether PCOS causes obesity, or vice versa, or whether they simply tend to occur together.