Adoptables info?

As I've been a bit out of the loop for months, I was wondering if I could get a bit more information on these adoptables that I'm seeing everywhere.  The majority are of the template variety, with mostly identical lineart and different colors/hair/markings.  The descriptions usually say something along the lines of "$5 each, tell me which you want, they are only female!" 

From an adopter standpoint, I'm confused as to what the purpose of "adopting" is and what can be done after adopting.  Are you buying rights to a character design or just a pre-drawn picture (that virtually anyone can save off the website)?  Please also give any other information or experience you have from this side.

I'd also like more information from the artist side.  Is this a viable form of income?  Is this what you would consider an honest form of income?  Is this worth looking into for me as an artist, or this likely a fad that will soon blow over?

I admit I'm sick of seeing them fill my inbox, but not really knowing how it started and not understanding them only makes me more annoyed, and curious.  This is my first time posting in this community, so if this is too off-topic, I apologize.

 PS Mods please tag, thank you =3