It's called [***litotes***][1].

From Wikipedia:

> In rhetoric, **litotes** is a figure of speech wherein **understatement is used to emphasize a point by stating a negative to further affirm a positive**, often incorporating double negatives for effect

> For example, "She's not bad looking" could be used to express that someone is gorgeous. Or it could convey that she's not particularly ugly, but also isn't particularly attractive.

> **The degree of emphasis depends on the context in which it is used.** 

From [Oxford][2]:

> **litotes**: <sub><sup>**[MASS NOUN]**</sup></sub> Ironical understatement in which an affirmative is expressed by the negative of its contrary (e.g. I shan’t be sorry for I shall be glad).


  [1]: https://en.wikipedia.org/wiki/Litotes
  [2]: http://www.oxforddictionaries.com/definition/english/litotes