Musar means the books of Ethics of the Middle Ages which focus on character development and fear of God.
The aspect of Musar that is the most important is the lack of "Shtick."
I mean to say, there is the basic message of what the Torah says to do--which is contained in the written and oral law--the Tenach and two Talmuds.
But the Middle Ages were unique in decoding that message.
And after the middle ages an onerous amount of garbage got mixed in to what is supposedly authentic Torah.
[Thus I avoid all religious groups because they teach pseudo Torah. The only kind of religious place I would be willing to walk into would be a Litvak Yeshiva. (These places are roughly based on the path of Gra and Reb Israel Salanter.)]

[The basic approach is that what ever the Torah says that is what we believe. There is no emphasis on doctrine but rather learning and keeping Torah, and it is God centered, not man centered. This is what makes Litvak Judaism unique-there is not idolatry of human beings, but rather worship of God.]

[The truth be told the basic set of Mediaeval Musar books does the best job of giving over the essence of Torah, that is the Obligations of the Heart, Gates of Repentance, אורחות צדיקים ,ספר היראה המיוחס לרבינו תם]

[I am not uniformly against Musar based on mystics. Mystics like the Ari and Avraham Abulafia I have a lot of respect for. It is just after the events of 1648 A.D. that the Sitra Achra {Dark Side} got mixed up with almost all religious Judaism. So books written after that period tend to lead people down the road to hell.] 

The idea of getting which books form a legitimate part of Torah is important, and excluding the books that pretend to be part of this tradition but in fact are promoting an agenda is important. The word אפיקורוסות heresy is a harsh word but useful. Every group has defining beliefs.  If it would not then it would not be a group. As distasteful as the word can be, it is  normal and inevitable in the process of marking boundaries, drawing lines of exclusion, and defining group identity. The term marks the most important boundaries of a group, beyond which a group understands its own identity to be profoundly harmed or compromised. It is a key flag in trying to determine how a group perceives its fundamental essence. All groups, religious or not, have boundaries. Without boundaries of some kind it would be impossible to have a sense of group identity. Granted, religious boundaries often make claims to truth, but these are hardly more exceptional than claims made by ethnic groups or political parties. Religions, when speaking of heresy, are simply doing what groups do generally.