Yeah, they are very small robots.
Manhacks are basically sharpened frisbees with an engine to keep them in the air - low damage, but quite difficult to hit in melee. These are very much the most common type, as they can be found loose in labs, and the can also be dropped by scientist zombies.
The other hacks are all released by grenadiers/elite grenadiers, and the all fly at their enemies and explode. Teargas hacks, flashbang hacks, they aren’t TOO bad, but grenade hacks can ruin your whole day, and don’t get me started on C-4 hacks.
Mininuke hacks are, luckily, VERY rare - the elite grenadier can launch just one at you. Those are bad things. Make sure you have a ranged weapon that you are a VERY good shot with - don’t want that one going off anywhere you can see it.