What is happens to my muscles when I'm sore?

I've heard that being sore means you've got small tears in your muscles. Is that true? I want to know what's happening to my body while I am stretching too much or am forcing myself to be more flexible. What are the ups and downs of being sore? and is it good to stretch while being sore? I've also heard that while the muscles recover from being sore they become more stiff. is that true?