Keyboard controls for pop-up prompts

Often when a prompt pop-ups on my screen (such as when I choose "shutdown" or delete a photo in LR) one of the choices button is shaded blue, and one of the choices button is outlined in blue. I'm never sure which of these is actually the highlighted/selected choice. What are the keyboard controls for this? Does is differ if I push space vs. enter, or tab through the choices? Did I do something to cause this?

Thanks for the help!
Brian

The blue one is the highlighted one and if you hit Enter, that's the button that will get selected...

Regards

The button that is fully highlighted can be pressed by enter.
The button that is only outlined can be pressed with space.

It's called something like full keyboard control and it can be turned off so you only get the full highlights. You can tun it on/off by pressing control+f1 or somewhere in the system preferences. Although I've had some problems using the shortcut so I would recommend checking out the preferences.

Edit,
I should clarify and I googled to give you a better answer.
Best to just go to System Preferences/Keyboard/shortcuts tab and select the option you want.
By pressing ctrl+f1 you disable/enable the whole Full Keyboard Access
By pressing ctrl+f7 you change another setting form text boxes to full control
So it does get confusing