Yogurt is a pudding-like food made from milk fermented in a culture of (typically) Lactobacillus delbrueckii bulgaricus and Streptococcus salivarius thermophilus. At the end of the process, the yogurt is usually heated to kill the bacteria. "Live active culture" is a type of yogurt that has the live bacteria still in it. Some people believe that this "live" yogurt has special health benefits.
Lactic and other acids produced during fermentation give it a slightly sour taste. It is usually eaten with fruit or jam.