All depends of the context, for example, you can use "sobre" as about, but also you can use it as "above" and it has a third meaning; you can use "sobre" as  envelope. I know, it sounds really difficult, in fact I find a little bit difficult to explain it. Here is a few examples:

The class was about numbers-"La clase fué sobre números"

The cloud is above the hill . "La nube está sobre la colina"

The letter has come in an envelope. -"La carta venía en un sobre"

I don´t know if I explain myself, I do my best, it's hard to explain even for native as me. 

And in all cases that you use sobre you also can use "acerca" or "se trata" this two only means "about" and are more formal, and probably is more easy to you use this.

Well, I think that is just as you want to say it, and I use indifferently both of them. For example

El libro trata de un mago oscuro

El libro es acerca de un mago oscuro

Te voy a decir algo, es acerca de una pelicula que acabo de ver

Te voy a decir algo, se trata de una pelicula que acabo de ver.

Sure, maybe in some special cases It would sound better one of them, but it's not so common to differentiate them frequently, I'm thinking right now in an example and nothing comes to my mind.

"acerca de" and "sobre" are synonims. (notice that everytime you use "acerca" you must use next "de") you use they when you want to say "about" 

but in fact, "se trata de" is composed with the part "se trata" and the part "de"
"de" in HERE is also a synonim of "acerca" y "sobre" 

so it is ok to say:
el libro se trata sobre la vida de Harry Potter (the book is about the life of harry P.)
el libro se trata acerca de la vida de Harri Potter
el libro se trata de la vida de Harrý Potter. 


the words "se trata" means you are particulary going to talk about the characteristics  of something as in the example above. 
buy you can remplaze it to talk about other things as:

Maria habló sobre su novio
María habló acerca de su novio
María habló de su novio. 
(maria talked about her boyfriend)

anhoter example with a question:
the question in english is
...well , i dont know how to write it in english, but is somethin like this:

About what the president talked? 

and in spanish is:
¿sobre qué habló el presiente?
 ¿acerca de qué habló el presidente?
¿de qué habló el presidente? (i have the presentiment this is a bit more informal, but is just my opinion)

so yes, "acerca de", "sobre" y "de" can be used indistintic. 
anothers words with same meaning are:
"referente a"
"relativo a" 

but they are very formal and has a  bit diferent estructure , example:
el libro es referente a H.P.
el libro es relativo a H.P.
 
there is a list of the synonims that i said
http://www.wordreference.com/sinonimos/sobre
you are interested in the second line.