The first 2 are ships that works reducing or increasing its fleet size at the other players scanners.
So if you have a fleet size 10,000 and 1 cloak ship, it will appear as 9,500, causing the enemies scanners to detect it later than it should at normal size (9500 seconds befor landing, not 10k)
And if you wish to scare them with a fleet size 7.5m, instead the real size 5m, add 10 Holo Ships
The Cloak Generator is a defense structure for your bases, it works similar to the Cloak Ship, by reducing its detection. The scanners will only list your base at their screens if their Stellar Cartography Tech is at least equal level of your Cloaking Tech.