What is dissent and heresy, and can a person be charged with embracing a dissenting ideology?

Dissent from Church teaching is never legitimate, but it is not always heresy.  Heresy is denial of dogma, a doctrine that the Catholic Church professes to be revealed by God, such as the resurrection of Christ.  Heresy is a very serious charge.
 

To say someone publicly dissents from ordinary Church teaching (such as prohibition of artificial contraception) is serious too, but not as serious as heresy.

The only umpires in the game that have authority to determine what is fair (orthodox) and foul (dissent and heresy) are the bishops and the Holy Father.  That's their job.

In other words, the judges are the bishops and the Holy See.  Individuals or groups can bring accusations like a lawyer, and they may believe they have evidence to back it up, but the only authoritative judges are the bishops and the Holy Father.
 
That does not mean that you can't steer people away from a book, a movement or a program that you believe to be less than faithful and sound.  I just would be careful not to go around saying that a specific movement, book or person is guilty of dissent or heresy, when the bishops and the Pope have not made a pronouncement.

For more information about heresy, check out the EWTN show, "The Great Heresies".