What is Google SSL beta?

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I am no internet expert so what is this new Google SSL beta? How does it differ from the classic one? What is SSL in the first place? I just tried to log in in google, and when i look for the Gmail link to look at my mail, i can't find it. LOL! stupid me, i guess.. But I was able to go into my mail, thankfully. haha. So what is SSL? thank you!