Is it loshon hara if someones speaks to you about someone that you do not know? Its still derogatory speech? Does the person speaking saying loshon hara?
This type of speech does not fall under the formal category of lashon hara, which is defined as denigrating or speaking badly of a specific and known person, in a way that will cause him damage, or lessen him in the eyes of the other party.
If the other party does not know the identity of the subject, and will not find it out, the prohibition will not be transgressed (see Rambam, Hilchos De’os, Chap. 7).
However, the underlying concept of lashon hara urges us to think positively about others, and to avoid thinking of them in a negative light, so that it is certainly a positive practice to refrain from speaking negatively about people, even if their identity is not known to the listeners, unless some positive result can be reached by doing so.