What specifically are you looking for that you aren't getting from all the other threads on here about it? A basic definition of what a secondary is?
The traditional definintion of a secondary is a vessel used to clear, or bulk condition beer, or to add an andjuct such as fruit, hops (though many dryhop during extended primaries) or oak to the beer. Or to Lager beers in.
Besides the post which atom posted, the debates about using it as opposed to long primaries have been done to death on here....If you're looking for more of that, I suggest you read THIS thread, it's become the "uber discussion" on this topic.
To Secondary or Not? John Palmer and Jamil Zainasheff Weigh In .
Most everyone on this forum has already ventured their opinions on the subject many many many many many many many many many many times, and most of them have ended up in the above thread. If you really do want opinions, and even some facts and citations, and articles, podcasts and other things on this topic, hit that thread, then make up your own mind.
What else are you looking for/ Every day there's at least 2-3 threads about whether or not to use them, and what folks do....it's not like it's an undiscussed subject on here, if you need something more specific what is it?