Disney Dollars are just like US dollars, only they feature Disney characters rather than presidents. $1 US = $1 Disney. You can get your Disney dollars at Guest Services (not all locations have them, but City Hall on Main Street is a guarantee). You can also buy them at most, if not all, Disney Store locations.
While you're on Disneyland (or Walt Disney World) property, you can use Disney Dollars just like cash. You will received US dollars as change (example: you purchase a $10 item with a $20 Disney dollar bill... you will receive $10 US in return). They're really just for fun. Alternatively, you can purchase them to keep as souvenirs. Different denominations feature different characters and you may mix and match as you please.
As for what's new at Disneyland, here's a list just off the top of my head (you may have already experienced some of these):
*Finding Nemo submarines
*Space Mountain 2.0
*Pirates of the Caribbean now features significant elements from the first two films
*Haunted Mansion has been considerably updated
*Tom Sawyer Island has been converted to "Pirate's Lair on Tom Sawyer Island", complete with a mini pirate stage show
*Disneyland: The First 50 Magical Years
*When I was there a couple of months ago, there was some work being done to the first floor of Innoventions; I'm not quite sure what was going on there but you might want to check it out
*It's a Small World is closed for a while while some extensive refurbishment is done
*There's a pretty cool tribute exhibit to Snow White in the Animation Building. Supposedly this is leaving soon (or perhaps already has), and I've heard that a tribute to Sleeping Beauty is coming at some point.
*Monsters, Inc.: Mike & Sulley to the Rescue
*Pixar Play Parade (different from the previous Block Party Bash -- and much better)
*Toy Story Midway Mania (officially opens 6/17 but will likely be unofficially available to the public before that)
Hope this helps!