Common Mistakes - By or Until

Until tells us how long a situation continues. If something happens until a particular time, you stop doing it at that time. (Until nos fala sobre a duração de alguma atividade. Se algo acontece "until" um tempoo em especifico, significa que você irá parar naquela data).

For example:

They lived in a small house until September 2003. 
(They stopped living there in September.)

I will be away until Wednesday.
(I will be back on Wednesday.)

We also use until in negative sentences. (Usado também nas sentenças negativas)

For example:

Details will not be available until January.
(January is the earliest you can expect to receive the details.)

If something happens by a particular time, it happens at or before that time. It is often used to indicate a deadline.(Se algo acontece "by!" um tempo especifico, acontecerá neste tempo ou antes. É sempre usado para um indicar uma linha de tempo) 

For example:

You have to finish by August 31. 
(August 31 is the last day you can finish; you may finish before this date.)

We also use by when asking questions. (Usamos o by em perguntas)

For example:

Will the details be available by December?
(This asks if they will be ready no later than December.)

http://www.learnenglish.de/mistakes/byvsuntil.html

Exercises: 
http://www.usingenglish.com/quizzes/8.html
http://www.grammarbank.com/by-vs-until.html
http://wpeau.ca/exercise/ex72.2.php
http://www.tolearnenglish.com/exercises/exercise-english-2/exercise-english-65900.php