I think LambeauSam put it all very well. But I think the first thing you & your husband should do is go to a few agencies' orientations (Private & state). They'll tell you what to expect, the problems the kids may have, issues you might face while fostering, what you have to do to be licensed to foster, etc. Then, once you've been to a few orientations, you can compare the different agencies, what they offer (support, special programs, different amounts of compensation), what they expect of you, etc.
I wish I'd been able to get in touch with more agencies to go to orientations (I only went to 2). I was a bit frustrated with the one I'm with, but I'm feeling better about it now.
I think the biggest challenge is that so much is out of your hands as a foster parent. There are things you decide for your own child that you can't do for your foster child -- like getting their hair cut without permission. (My current headache, as my fs is 20 months old, AA and constantly puts stuff in his hair. It's long, & while I'm not a fan of braids on boys, it's about the only way to control his hair.) If you want to take a fc on a trip, you have to get permission. And, of course, their ultimate fate (whether they go back to bioparents or not) is completely out of your hands.
It's also a little unnerving (although totally necessary) to know that cws can stop by at any time, with or without notification. It hasn't happened to me (in fact, mine didn't show up when she was supposed to), but I'm aware that it can.
All of that said, it's a very worthwhile endeavor, if your family is up to the challenge.
