The expression is mostly used to show disapproval. When someone crows about something, he talks very proudly about something that he has done. He is rather vain about it, and you find it very annoying.
Example: A politician is always crowing about his/her achievements.
It’s also possible to say ‘crow over’.
Example: Some of the Australian newspapers didn’t crow over their team’s achievement.
Source: “The Hindu” – Know Your English Column – Jan 15, 2008