Vaginal yeast infections are caused by overgrowth of fungal organisms, usually the Candida Albicans species. Symptoms are usually itching, burning, and white, cottage-cheese appearing discharge. They are caused by killing off the "good" or "healthy" bacterial vaginal flora which allows the yeast to flourish. Recent antibiotic exposure is a common predisposing cause for yeast infection, but many women experience yeast infections without a specific cause.
Bacterial vaginosis is an overgrowth of one of the normal bacterial vaginal flora to the point that it becomes pathogenic and causes an infection. Common species are Gardnerella and Bacteroides. Typical symptoms are discharge that can be yellow, grey or greenish with a fishy or foul odor, and vaginal burning. Treatment is typically with an antibiotic preparation to be taken orally or intravaginally. Unfortunately the oral antibiotics can then cause a yeast infection!
Since symptoms of both can somewhat overlap, it is always best to see your doctor for the correct diagnosis instead of trying to treat symptoms without a sure diagnosis.