They are all over the net- try searching them in Google, even on Deviantart and Planet Renders

They are basically things you add in your signatures for the background. There are many kinds- the most common ones are the effect c4d ones, they are common among sigs. Unfortunately, they can be overused at times, ruining the outcome =\

Hikari has got you covered; they basically are things you put in your sig that can make it flow or make the background better.

Its a thing to basically use somebody elses work...

3-d render to paste on your art to make it look better...