The Faces of "Tratar"
Many native speakers use this verb incorrectly.
The expression se trata de should be followed by a noun. That noun can refer to a person, thing or a matter or topic at hand. Note that since it is an impersonal construction, that noun (person, thing or topic) is not the subject of tratarse. It's roughly equivalent to saying It's about... or It has to do with... Note that in English the word it is the grammatical subject -- and Spanish has no equivalent for it as a subject.
Se trata de una persona desamparada (It has to do with a homeless person).
It would be incorrect Spanish to say, instead of the above: La persona se trata de una desamparada. Instead, it should be, if it is important to make persona the grammatical subject: La persona es desamparada.
Many native speakers also confuse the use of tratarse de with that of tratar de. The former was explained above. The expression tratar de is used to tell what something is about. Note that since libro (book) is the grammatical subject, the construction ought not be reflexive and so se is not to be used:
¿De qué trata el libro? (What is the book about?)
It is possible to use tratar in a few other ways. It can mean to deal with or treat (people), in which case either the preposition a is used (personal a) or tratarse can be used -- with con, meaning to have dealings or association with someone:
Ella trata bien a su esposo (She treats her husband well).
Juan se trata con Alberto (Juan has dealings with Alberto).
Another option for the second example is Juan tiene trato con Alberto. The noun trato can also mean a deal, as in the common expression, followed by a handshake: trato hecho -- it's a deal. The expression Tiene buen trato means, roughly, He/she has a good way with people -- as in, a good touch when dealing with people.
When tratar is used with de and followed by an infinitive, it means to try and is an equivalent of intentar plus an infinitive:
Juan trata de aprender inglés (Juan is trying to learn English).
Juan intenta aprender inglés (Juan is trying to learn English).