A Mafia Goon is a vanilla member of the Mafia. This means that they have all the abilities that a Mafia member would normally possess, but no abilities beyond that. In most games, these abilities include knowledge of who the other Mafia members are, the ability to communicate with them overNight (and sometimes during the Day too), and the ability to kill one player each night as long as no other Mafia member is performing the kill.
If a Goon is scanned by a Role Cop or similar role, the Role Cop will receive a "vanilla" result (not "Goon"). Likewise, Mafia Goons show as vanilla to a Vanilla Cop, can be affected by Simple roles but not Complex roles, and so on.
A Goon inherently cannot be given any role modifiers or any additional roles whilst remaining a Goon – that would, by definition, make them not vanilla. For example, modifying a Mafia Goon to be Bulletproof would produce a Bulletproof Mafioso, not a "Bulletproof Mafia Goon"; the latter name is self-contradictory.
In a Normal game, there is no way for a Goon's role to change. However, Goons are not inherently resistant to becoming non-vanilla; role-changing roles like Inventor can grant Goons extra actions as easily as they could grant actions to anyone else.
The term "Goon" is only ever used for members of a groupscum faction who have the faction's basic factional abilities, but nothing else. Although this is normally the Mafia, the term "Goon" is sometimes used to clarify that a Werewolf is vanilla. The same naming convention could also be used for weirder groupscum factions; in every case, the Goon has access to all the faction's basic factional abilities (whatever they are), but no powers beyond that.
Use & Balance
The vast majority of Mafia power roles exist purely to weaken or frustrate the Town's power roles. In many cases, you will want the Town power roles to be at their full strength; even when the Town needs weakening, it would be rare to weaken it by making every member of the Mafia a power role.
Goons provide the Mafia a way to increase their numbers without needing to include additional power. As a consequence, they are very, very common; in most setups, at least half the Mafia are Goons, and it is far from unheard of for the Mafia faction to be consist entirely of Goons. Generally speaking, if you want to fill out the numbers of your Mafia faction, you should do so with Goons unless there is a good reason not to.
Most active power roles need to use their night action for something else (unless they are Multitasking), so in practice the Mafia's factional kill is most commonly performed either by a Goon or by a Mafia member with a passive role (using a Goon also somewhat reduces the damage if you get caught performing the nightkill). However, the loss of any Mafia member is a major blow to the faction, so Goons should not be seen as significantly less valuable than anyone else.
Sample Role PMs
This part of a role name serves merely to denote that the role is vanilla, and does not add any special powers that would be visible in a role PM.