Fandom is defined by meticulous care and really giving a shit and having strong opinions and arguing. It’s a community with opinions. If they were in a room they’d be fist-fighting some of the time. You know, the Sam-girls and the Dean-girls would be like pulling each other’s hair, some of them! That goes with the territory, and it’s also immensely rewarding because they’re giving a shit and they’re paying attention
Since the beginning of the show, some fans have had a strong allegiance to either Sam or Dean. Sam girls are devoted to Sam and will protest any perceived slights against his character by other fans, Dean or the writers. A common complaint is that the story is too focussed on Dean at Sam's cost. Dean Girls of course protest the opposite.
The most obsessed Sam Girls are known as ESGs - Extreme Sam Girls
Becky Rosen is a canonical manifestation of a Sam Girl, being intensely focused on Sam and dismissive of Dean.