Holo Discs can be found lying around in the Future or found in Buy Mode > Sort by Function > Study > Indoor Activities for a few hundred Simoleons.
The Holo Disc confused me at first, when I received one as a reward from the Time Traveler and couldn't find it. This device emits a pet of sorts that has a mood, can be talked to, and its forms exchanged between Sims. The Sprite from the Holo Disc allows you to fill social need wherever you are while maintaining it (clean Sprite) and talking to it will give you Advanced Tech skill experience. There are 11 total Sprite forms to discover. You gain 3 by Leveling (start with Feronix, Futech at 3, Zigrid at 5, and Chromec at 7).
Talking to other Sims and getting friendly with them, you can exchange Sprite Forms and possibly gain a new design for your Sprite. As you'll see below, there is a Skill Challenge to collect all the Sprites to gain a bonus one for your Holo Disc emitter. Do let us know any special effects Sprites may have so that we can share them with other readers!
You can Summon the Sprite to follow you around, and Sprites can chat between each other. Sprites that are in a bad mood are usually red, while other colors indicate better moods.