I am new to Arduino. 
I have been trying to connect a 7 segment display (3 digits) to my Arduino and I was successful, but I am not doing any connection to ground. I do not find the data sheet so I am not sure if this connection is well done.
My doubt is: Is the connection to ground necessary? Or how do Arduino pins work? I am using 12 pins of the Uno to do the set up.

Thanks

My set up is the following:
![enter image description here][1]


  [1]: https://i.sstatic.net/ADNED.jpg