Now you can add anyone to new or existing wave even if he is not in your contacts. An invitation will be sent to him to join Google Wave with the description of wave.
The APIs stuff is just making life easier.
Gadgets are like small applications we play with at different platforms.
Google already have gadgets platform available at iGoogle. Now google introduced Gadget Ads for Adwords. Now you can have more interactive ad campaigns with gadget ads.
