Wikipedia actually has the best definition of Mainstream media that I could find: MSM is a term and abbreviation used to refer collectively to the various large mass news media that influence many people. It both reflects and shapes prevailing currents of thought.
Examples of media NOT considered mainstream would be user-generated content such as YouTube videos or blogs. These are published by individuals instead of the major corporations that publish newspapers and broadcast shows.
As we all know, journalism has changed a great deal over the years. A couple of decades ago, journalists prided themselves on being unbiased and kept their opinions out of their news broadcasts and newspaper articles other than specific editorial pieces. Today, each reporter seems to have their own agenda and political slant when reporting the news. The jury is still out on whether this is good or bad but it is increasingly hard to find “just the facts” on anything.