First of all, when it comes to media, I am talking about the news media.
We all know what the mainstream media is. Your typical cable news and corporate news networks, establishment newspapers, news magazines and all web content pertaining from sources therein.
The alternative media (or progressive media) relates to online sources that place themselves outside of the regular media due to a differing standard of reporting. The idea is basically reporting the news as it is, without filtering details though the lens of a given political or corporate ideology.
Some alternative media sources stick mainly to societal and political issues, others to those concerning the environment. Some of the sources overlap in these areas.
While all think that they are doing the job of journalism better then their traditionally established mainstream counterparts, this is not always the case. Its certainly an improvement, but sometimes its hard to miss that the ideological slant of progressive reporting is just reversed to the well known slant of some traditionally slanted media sources. An extreme lean to the left instead of to the right.
I do not hate alternative or progressive news media, nor do I hate any other source of news media. When browsing the news (or sourcing links) I will use whatever articles that I find that best fit the situation. Many will be left leaning, but others may be from other sources. Its normally more about the information contained therein, then the way in which its contextualized by the author.
All definitions utilized in this explanation are my own categorizations (if they differ from yours, or those utilized by others).