Technically being a furry just means you like anthropomorphic animals. That's about it, and sometimes people role play (NOT IN THE SEXUAL SENSE) and just pretend to be animals online to make up stories with other furries.
That being said, "yiff" or "yiffing" can mean a lot of things, from anthro porn to sexually suggestive pictures people draw, or to people ejaculating onto stuffed animals, or having sex in fursuits.
NOT ALL FURRIES ARE YIFFERS.
Not all furries consider this to be equal to sexual orientation or gender identity or race.
HOWEVER, everything varies from person to person.
For myself, I consider myself to have a human body, but inside I am an anthropomorphic vixen with gray angel wings and purple fur around my face. Obviously, I know that's not the case for me physically, and no, this doesn't mean I have psychological problems.
I consider it to be like species identity, like "If you could be any animal in the world" but deeper. It's a part of who I am, and it's how I feel inside. I don't have a fursuit yet, but I want one so that I can feel more connected to what I feel like inside.
This is a very personal part of me, and your friend might not want to talk about it. You can ask her, see just how deep the rabbit hole goes, but no matter what you have to respect what she tells you. If she doesn't want to talk about it, drop it.
For some people, it's a really kinky fetish, like people who wear dog collars in public and are walked around by their masters. So you can understand why she might not want to tell you everything.
This comic kind of explains it best:
http://www.michiganfurs.com/forum/index.php?topic=7915.0Scroll down until you find it. Also, read that page. It'll help you understand.
We are not freaks, weirdos, perverts or pedophiles. To each their own.
Also, don't confuse this with zoophilia or beastiality. People who like anthro and yiff are not always sexually attracted to animals, and we will not have sex with them and never plan to.
Don't forget, with every minority the people who take it waay too far are the ones people will pay the most attention to. They do NOT speak for us.