My cousin has fostered quite a bit in this state. They do teenagers. Their first experience was pretty horrible and they swore they'd never do it again. They had a teenage boy who stole from them, was pretty angry and violent and doing drugs. That was really hard and eventually he got moved to a new home.
They went without any fosters for a couple of years until the state called and begged them to take another. His wife kept saying no, but the social workers wheedled her into it. lol. This was a very different experience. They got a 15 year old girl this time who was living with her alcoholic dad. He was physically abusive to her and eventually lost custody. She's a really sweet kid, got along great with their two teenagers. Really shy and introverted but after being with them for several months came out of her shell quite a bit and became a regular part of the extended family.
They basically informally adopted her. After a couple of years she was legally emancipated and could have gone off and rented her own apartment if she wanted. But she was welcome to stay with them and did. She's a great kid and we're really glad she was able to stay in the family.
Each kid is really different. Each one has different struggles and a different background, different ways of coping. I think the hardest part would be having to let them go when it's time to switch homes, and that's something I've heard from a lot of foster parents because you get really attached.
In WA you take classes, they do background checks, evaluate you and eventually you're a certified foster parent. You can specialize in certain age groups or types of cases. I know of one lady who is on call for drug addicted premature infants, she has a ton of experience and training with that specific group. You're also required to have a separate bedroom available for the foster child so they aren't expected to double up with a bunch of other people.
From what I can tell, here at least they just randomly call you with emergency placements so you don't get much warning. There's one lady in our church that is a psychiatrist who works with the police force. She's a certified foster parent. The officers brought in a 3 year old boy because his parents had been arrested for something heinous (I forget what) and she ended up taking him home straight from the police station because there wasn't anyone else available. He was badly neglected and they eventually signed over their parental rights so she ended up adopting him too. Then she fostered and adopted a group of four sisters that had been abused. She was really well equipped to help them through their emotional difficulties though.