Doctor Who/Torchwood/The Sarah Jane Adventures Crossover Guide

Doctor Who has spawned three spin-off series' since its return in 2005. Doctor Who sits at the heart of the franchise, with The Sarah Jane Adventures, Class and Torchwood catering for the children, teenager, and adult markets respectively. The question is: how does it all fit together? Below is a chronology of the show's main crossover episodes. It's by no means exhaustive, but what follows should be enough to give the new viewer a rough idea of what to expect, and the order in which to expect it.
the franchise, with

Doctor Who: The Wedding of River Song

Churchill: 'All of history is happening at once.'

You have to admire the ambition of tonight's episode—for fifteen minutes I just stared at the screen, both amazed and befuddled. The visuals were at times breathtaking: from the car carrying balloons, to steam trains exiting the Gherkin, to the vaguely impressive pterodactyls. (Less impressive after Terra Nova's, sadly.) I even liked the weird meld of the Tudor/Roman/post-war eras. True, we ended up with more questions than we got answers to, but this was a remarkable ending to what's been a wholly remarkable season.