DD? DS? Other things on this site?

Ssneed645

I've been seeing a lot of abbreviations on this site, and I am clueless to them all. What does DD and DS among other abbreviations on this site mean? Anyone to help a terrible computer lingo-ist? Sorry, I'm aware that this is completely unrelated. I just figured other 1st trimester people would have asked this before and may be able to tell me.