It is a comedy series, about the everyday life in Brooklyn's 99 precinct. The main characters are 2 in my opinion: Jake Peralta (Andy Samberg), an almost cartoon-ish talented and carefree Detective, and Ray Holt (Andre Braugher), the very serious Captain who just took over the 99th precinct.
There are many more characters included of course, but these 2 are the "heart" of the series.
The first season consists of 22 episodes, and the second one of 23, and each episode is almost 20 minutes long.
I found the first season funny enough, and I watched it rather fast. But the second one was not so good, and I think it was kind of a boring one. In both seasons are included some really good episodes, of course, and from time to time it's lots of fun watching them. Some others are less inspired and kind of boring though, but I guess that is something normal.
The 3rd season has already started, but I can't say that I'm so impatient to see it. Maybe I will, maybe not. Depends of what other series I'll have available.
Concluding this, I would like to say that 'Brooklyn Nine-Nine' is a fun and easy to watch series, but don't expect something really great.
That's why, my rating will be: Season 1: 7.50 / Season 2: 6.5 - 7.0
On IMDB the rating for both seasons is: 8.3
In the following link you can see a trailer in case you are interested.