Time to perform a miracle. (I hope you have a 2007 camry otherwise I have absolutely no clue what you are talking about)
D is actually anything from 1st-6th gear depending on speed and driving condition. When you slide the gear lever to the left your actually going into whats called Tiptronic mode. In tiptronic mode you are in control of what gears the transmission will be in, much like a manual transmission.
Manual VS Tiptronic:
In tiptronic you don't have to worry about letting off the gas and pushing the clutch to change gears. IN our case you simply push the lever forward or backward to either upshift or downshit respectively. Furthermore, tiptronic has safety features that will keep you from doing things like downshifting into 1st gear while going 80MPH. Tiptronic will also downshift for you if you are in too high of a gear (It will not show on the display but it will downshift). What this means is if you are in 6th gear and you slow down to a stop the tiptronic will actually put the tranny into first gear. When you accelerate you will start from first and then go up until you reach the displayed gear (in this example: 6th gear).
It is safe to switch between tiptronic and drive during motion. I personally do this whenever I want to accelerate without having to floor the accelerator. You won't damage the transmission by doing that.
PS: When slowing down to a stop while in tiptronic i recommend you wait until you are fully stopped to downshift into 1st (Ofcourse you don't even have to downshift if you wish because the tranny would already be in first as explained above). I also recommend you stay away from keeping your RPM's above 3,000 for an extended period (bad for gas mileage, you should upshift to 6th gear to lower the RPM's). Ofcourse if you are hitting 3,000RPM's in 6th gear you are A. going too fast
and b. have no other choice but to go 3,000RPM which isn't that bad for your mileage because your going so fast. From my experience i've found that it's best to up-shift at 2,500RPM and to go to 6th gear when you are cruising at a steady pace (I believe this is what the tranny would do if it were in D). And finally, odds are you'll be getting better mileage with D then you would with tiptronic. I haven't tested this out yet because i'm still breaking my engine in(currently at 500miles half way tehre!), but the only argument for getting better mileage in triptronic that i can think of is that you know what you're going to do and you could better chose the tranny gears then the transmission.
Goodluck & feel free to post any other questions (Ignore brandon haha)