Where in the Dragon Ball timeline is the Super series?
In the last episodes of Dragon Ball Z, the Tenkaichi Tournament takes place 10 years after Buu's defeat.

Pan is one of the tournament participants, at the age of 4.
Goten and Trunks are teenagers.

In Dragon Ball Super, Pan is still in Videl's womb.
Trunks and Goten are still kids.

You can say that Dragon Ball Super takes place before that tournament and after Buu's defeat.

Mind-blowing fact: Goku can already transform into SSG in that same tournament eheheh :D

EDIT: Here's the timeline from the Wikia:


  Dragon Ball Super
   Age 778
   Around August 18 Beerus awakens from a
  slumber and searches the Super Saiyan God.
   Bulma's birthday party is
  held on a cruise ship.
  
  Dragon Ball Z
  Age 781, May 7 
  The 27th World Martial Arts Tournament takes place. Mr.
  Satan wins. Buu comes in second place.