Foster Families: Are there such things as "long term" foster parents?

I’m pretty sure most foster parents only have the foster child for a short period of time, but if the children’s homes are overflowing, and nobody is adopting, would they have kids periodically switch from home to foster parent, making room for new kids etc.? And if you are a foster parent, how long can you have a foster-child without adopting them? (I’m guessing it all depends on the laws, the child’s situation, etc., but I have no clue!)