Intruders fall into two categories: undercover spies and agent/saboteurs from the guild of Assassins. Skrashers could be considered a third type, but unlike the other two, they didn't come to your station on purpose. Security scuzzers will open fire on these three, even if you have set them to ignore all criminal activity.
In some games you can hire assassins and saboteurs to attack your enemies.
They looks like normal peeps, they occasionally kill a peep and are rumoured to indulge in espionage and sabotage. You can tag them with a cross-hair if you happen to put your pointer over them. It seems they can be found out in other ways, such as if they are caught committing a crime.
Dressed in black they creep around and plant Bombs. They must be tagged with a cross-hair to be targeted by security.
These agents have far higher health that any peep and will usually kill a single peep sent against them. They are best attacked by multiple peeps and Security Scuzzers.
While Intruders mostly spend their time sneaking around, they have their own dance animation if they enter a disco.