Another method is using Symbols.

Say you have an object that you've created.  In this case, I made a smiley face.

[![smiley][1]][1]

With entire object selected, open your Symbols Palette and click the New Symbol button.

[![new symbol][2]][2]

Configure your symbol's options and click Ok.

[![symbol options][3]][3]

This adds your object to the palette, as a symbol.  You can then drag out as many copies of your newly created symbol and transform them as you please.

[![symbols before][4]][4]

To edit your symbol, double click on its icon in the Symbols Palette and it will behave the same as doing that in PS does with smart objects; you will see that AI opens your symbol in Isolation Mode.

[![blue eye][5]][5]

You can make whatever changes you want to here.  In the case above, I made one eye blue.  When finished, you can simply double-click outside your object to exit Isolation Mode.  You will see your changes propagated throughout your document.

[![blue eyes][6]][6]


  [1]: https://i.sstatic.net/3D4Zx.png
  [2]: https://i.sstatic.net/5KRVj.png
  [3]: https://i.sstatic.net/57n1s.png
  [4]: https://i.sstatic.net/Spfvq.png
  [5]: https://i.sstatic.net/cTCTh.png
  [6]: https://i.sstatic.net/76p3C.png