Acidophilus is a type of probiotic. Probiotics
are "good bacteria" our intestines need to function properly. They are there to help flush out bad bacteria. If you ever get food poisoning or the stomach flu it means your gut is overloaded with bad bacteria, it will help you get over the sickness faster if you replenish the good bacteria (either by taking supplements or by eating A LOT of yogurt). Same goes for diarrhea... When you have D your body is trying to eliminate the intruder that is wreaking havoc; in the process of eliminating the bad your body is also eliminating the good. Adding probiotics will also help with D. Probiotics are also great for combating yeast overgrowth (i.e. thrush & vaginal yeast infections). Probiotics will also help boost the immune system and reduce cholesterol.
There are a bunch of strands of probiotics; the most common are Lactobacillus, Acidophilus, and Bifidobacterium.
I take probiotics daily via supplements and yogurt. I eat 1 Dannon Activia yogurt a day (high probiotic count), 1 Culturelle pill (supplement) and 1 Kyo-Dophilus pill (supplement). This routine has worked wonders for me. I am much more regular and have far less trouble with the "bad" foods and much less bloating. By the way, you can't overdose on probiotics. Your body will just flush out what it doesn't need.
I hope that helps, Sarah. Please don't be shy about asking questions.