Most rescues have clear guidelines for foster carers -you can often find them on their websites. Generally, when you volunteer, the foster co-ordinator will find out what you are interested in - short, medium or long term fostering. Short term could be as little as one night, but it's more typical for a foster placement to last for at least a fortnight while the dog is being assessed before being ready for homing. At the other end of the spectrum, we have three long term fosters - the rescue funds vet care but the dogs will be here for the rest of their lives. For two of them, they came in as emergency fosters when their owner was rushed into hospital - sadly the owner's health deteriorated to such an extent that returning the dogs wasn't possible, and as they are elderly and had been together for many years, it was felt sensible to leave them where they were. I must stress though that as foster carers we were very closely involved in these discussions and would never have agreed to it unless we were 100% happy.
While with you, you will be expected to provide care and possibly start basic training - things like loose lead walking for instance. The rescue will provide you with an allowance for food or even supply the food directly to you and will arrange with you about vet care - sometimes they will say which vet you need to attend as many rescues have accounts with particular vet surgeries. You will be expected to provide regular updates on how the dog is settling in and any issues that have come up, and things like photos, videos are very useful these days for websites and FB pages. Good rescues will provide training and support for foster carers and will always make sure you are matched with a suitable dog - if for any reason you can't cope, or have problems then the dog will be removed, but there will always be support and advice available.
Foster carers also provide a lot of care to dogs with medical/recuperation issues. Sadly, rescues often take on dogs that have been starved, neglected or are ill. This is a more specialist role where you will be working closely with vets, to make sure that once the dog is well enough/stable to leave vet surgery care, they can recuperate in a home environment. You might have to do things like look after wounds, feed very special diets, administer drugs etc - again, this is NOT something you will ever be forced to do - but it's hugely rewarding to see dogs that were in a dreadful condition get their health back and recover. To do this you need to be patient, observant, not too houseproud, and willing to work in partnership with your vet.
Foster caring is incredibly rewarding and a wonderful thing to do. You will meet some gorgeous dogs and there is no finer feeling than seeing a dog you have fostered happily settled in a home. Bear in mind though that most foster carers will "fail" at least once and end up keeping a foster dog permanently.
Good luck!