Let me share with you with the Department of Children and Family Services in the state of Illinois says:
Another need is temporary respite care. I first learned about this kind of care when working in the public school system. A few of my students were being cared for by foster families. Because these children had significant medical and/or developmental needs, their care was round-the-clock and very intensive. Most of the foster families could not call a high schooler to come babysit or ask a friend to watch their children while they ran errands, take children to medical appointments or enjoy a well-needed break. For that reason, respite care was necessary.
Adoption.com explains respite care like this:
Respite care is the short-term care of a very dependent or difficult child which enables the parents - birth, foster or adoptive - to take a break. Some children's needs require round-the-clock intensive care and parents can soon become burned out. Respite fostering is often very rewarding, and a close relationship can grow up between families over the years.
It has been known for respite carers to eventually be asked by a family to adopt their child. Some respite carers will have had special training to care for the needs of very handicapped or challenging children. This kind of foster care can be very suitable for people who need to know that a placement will be very short (but repeated at more or less regular intervals), because respite care will almost never be longer than a couple of weeks, and is very often measured in hours or days. Respite care is often a critical component of family preservation efforts.
Taken from: http://www.fosterparenting.com/foster-care/respite-care-2.html
A great (and short) article about preparing for respite foster care can be found here:
AdoptiveFamilies.com has a wonderful list of articles, books, and websites. Follow this link, http://www.adoptivefamilies.com/foster/, to read more.