Critical care is often used when your chin isn't eating, whether it's got an infection, teeth issues, or unknown causes. It's a brownish-powder that's mixed with water and syringe fed to the chinchilla in place of the food they would normally be eating. Since they're not eating, it's used to sort of force-feed them. There's regular and (I think) apple banana flavored (I use the regular, but I'm pretty sure apple banana is the other flavor). Critical care you can often purchase from your vet. Alternatively, you can get it from a member here - menagerie. It's good to have on hand, ahead of time, from when you need it. We keep a pound in the freezer so that we have it in case the need arises.
Lifeline is not the same as critical care. It's sort of an "addition" to critical care... I've never used it (though I need to buy it), so I don't have personal experience with it, but I know a lot of people use it when their chin is under the weather, and it can also be given when the chin is doing fine, just as a preventative. Critical care is meant to act as a substitute for food (and quite a bit of it has to be fed every day to the chin to keep their weight up), whereas Lifeline is more... (quoting the website) to "Speed Healing, Ease Stomach & GI pain, and Increase Appetite while it helps to fight the problems that cause symptoms such as infection, diarrhea, indigestion, nausea, and poor appetite." Very little of Lifeline is given, even to a sick chin. Here's where you can get lifeline --->
http://chocolatechinchillas.com/id6.html
Acidophilus is a probiotic - that means it's good bacteria (if a chin gets an infection - that's bad bacteria). Good bacteria fights off the bad bacteria, so ideally you want lots of the good bacteria. Probiotics, acidophilus typically the common one, are often used in conjunction with critical care, or around the same time an antibiotic is being used to treat something wrong with the chin. Could be used preventatively as well, I suppose. Acidophilus can be found at many health food stores, by the vitamins -- I get mine at GNC. At whatever point you get a probiotic, you want one with as high of a count of live bacteria as possible - like in the billions (what I have has 14 billion live bacteria). And if a probiotic is being given around the same time an antibiotic is given, you have to wait four hours after the antibiotic to give the probiotic.... otherwise the antibiotic will kill off the probiotic. Hopefully, though, if you're lucky, you'll never have to worry about all of this stuff. I wish you the best of luck and healthy chins. Sick ones aren't much fun...
Don't feel foolish for asking questions! We were all where you are at some point, and asking is the only way you learn!