A Feral ghoul roamer is a Feral ghoul with greater damage protection. It is also faster and does more damage.
Ghoul Roamers wear armor, unlike their cousins. It stands to reason that these ghouls may have turned feral at a slower pace. Their armor resembles the Combat Armor worn by all instate U.S. soldiers, mostly common to the National Guard and US Army.
Roamers are usually found in places where there are usually ghouls. However, they are less common than standard feral ghouls. These ghouls are stronger and tougher than a standard ghoul, but still fall relatively easily compared to most other creatures out in the wastes. However, they do have a tendency to appear in small groups and can be dangerous to a low-level player. The best advice for dealing with them is to aim for the head and/or cripple their legs with the Dart Gun.
Feral Ghoul Roamers have marginally more health than most normal Wasteland humans, and are also significantly faster, with stronger unarmed combat damage. However, unlike human NPCs, they lack functional body armor, and can be taken out relatively easily with 2 to 3 hunting rifle headshots.
These enemies can be as common as regular Feral Ghouls in many areas on Very Hard.
Because they wear Combat Armour they could be ghoulified American or National Guard Soldiers that helped people down into the Metros or fallout shelters, only to have radiation roll down the entrances into the underground areas.
Behind the scenes
"Roamer" is a term that came into recent use among zombie fiction in general and refers to a zombie that prefers to travel searching for victims instead of laying about rotting. The most prominent user of this term is the comic "The Walking Dead" by Robert Kirkman.