Dickgirls are summed up as hentai girls with penises. They can be from birth, magic, or even transexual. Dickgirls vary as the balls-out variety (e.g. normal male groin), no balls but penis, or penis and vagina. There are many different artists out on the internet, and there are even mangas and hentai games with dickgirls.
Dickgirl material includes anything from posing (no explicetly sexual activity), masturbation (1 person), sex with another person (male/female/dickgirl 1x1), orgies (such as fucking AND being fucked), and to any variant of hentai such as BBW, furry, tentacles, bondage, etc.
Another common term is futanari or simply futa. Dickgirl also refers to media of dickgirl(s).
A misconception about this might be that it is inherently gay, but it is not true since the majority of users are straight.
"You know that butch from that anime? I bet there's dickgirl of her."
"You know those two girls from that anime? I want to see that one fuck that one, but lesbian porn doesn't turn me on."
"I know just what you need. Dickgirls!"