The former lets users search across the body and comments of public posts. The latter searches across public profile info including fields such as name, bio, location, tag line, and description.
"Our first API release let you retrieve public posts," says Google+ API software engineer Jordanna Chord. "We’ve now added ways for you to see how people are publicly engaging with those posts -- you can find out who reshared a post or who +1’d a post, and you can read the comments on a post."
For that, developers can use the method people.listByActivity. Documentation on that here.
Developer use of the Google+ APIs will be key in making Google's social network more useful across a variety of applications. APIs certainly worked wonders for making Twitter relevant to more people.
Of course Google+ is really just part of the greater Google network.