Fangirls are women who are obsessed with media or brands, to the point they spend hours engaging with such media. It's a nice way to say, "female nerd".
Fanboys are the male equivalent.
Fangirls may often be femcels however, they are not aware of having such an identity nor do they identify as such. It may also explain some of their behavior involving attached and clingy behavior regarding characters/people they like. And also put into perspective the reason why many fangirls act in extreme ways that would be normally considered “creepy” and “perverted” if such actions were done by men. To some extent, this creepy behaviour is increasingly being done towards extreme fanboys.
Fangirls are fans of numerous different niches. These include: romance, "ships", yaoi (yaoi made for females), and numerous types of pop music. Shipping, also abbreviated, "ship", means imagining fake romances. Fangirls will, "ship", real people, not just the peoples characters or original creations (OC's). Often, such fandoms participate in creating fan-art on tumblr or twitter as well as fanfiction.
Many may also consider themselves left-leaning. Although, the politics of fangirls usually only amounts to justifying their outbursts, or trying to seem hip through insincere tokenism.
Fangirls also tend to have an obsession with, "shipping" two guys (real or fictional) together under the guise of, "le LGBTQ activism". This is just yaoi, not actual LGBT representation. Many are known to flood sites like twitter and Tumblr. Fangirls will also flood leftist sites and programs such as breadtube and to a lesser extent, discord, and leftypol. Fangirls often are the initiators of things such as cancel culture.
Fangirls often attack other fandoms that are consumed by men.
Their hypocrisy is that many of the content that fangirls consume is often not much less depraved than what the worst of the fanboys consume. An example would be how many fangirls teamed up on sites like Twitter to attack fictional characters such as Usaki-Chan and lolicons.
While many fangirls may act like these actions are fighting for a progressive narrative, this is often greatly disguising their obsessive disgust/jealousy for such fictional characters. When confronted with this, fangirls often call the accuser a "racist", "sexist", "pedophile", "incel", or "nazi". Fangirls also love to imply that anyone who ever criticizes them can only be one of such people. These are just insincere accusations, usually to deflect from their hypocrisy.