Everything. Google+ selects this category automatically. (You can tell which category is selected because its name is written in gray; the other categories’ names are in blue to indicate you can click them to switch to them). When it’s selected, the results you see are a compilation of posts, Google+ members, and results from sparks (explained in a sec).

People. Click this category to see a list of people using Google+ who have your search term in their name or profile. You see each person’s name, the occupation listed in their profile, a snippet of the first bit of their profile, and an “Add to circles” button you can mouse over to pull up a list of your circles to add this person to. (If the person is already in one or more of your circles, you’ll see a button indicating the name of the circle[s] instead.)