Google Plus posts are now part of Google Search results too i.e. if you enter a search string on Google then you might see results from Google Plus posts. Also if you happen to be logged into your Google account at the time of performing search then count of Google search results from plus.google.com can be much higher as Google will try to show you the information shared by your friends on Google+ or +1ed by them on the web.
Search Google Plus using Google (Without logged into Google account)
To search in Google Plus we will do a site specific search using Google’s advanced search operators. For e.g. If we have to search for “Hurricane Irene” on Google Plus, we would search using site: operator as shown below.
“Hurricane Irene” site: plus.google.com
Search Google Plus using Google (Logged into Google accounts)
If you type the same search query as above while being logged into your Google accounts then your search results might appear different and would have more results of the posts which have been +1ed on Google Plus by someone in your circles.
You might have noticed a slight difference in search results shown in above 2 screenshots. In the first case I searched on Google Plus when I wasn’t logged into my Google Account. So the results are shown based on a public search done on all indexed Google+ accounts. But in the 2nd case when I was logged into my Google account and performed the same search then it showed one post +1ed by Robert Scoble and another shared by Guy Kawasaki (in my circles), which I as a user would consider as recommendation from my someone in my circle. Probability of my clicking these recommended Google Plus posts is much higher than other general posts.
Using the above 2 search methods you can perform any search query on Google+ and get results for related content. Also note that Google can only shown results from posts which are shared publicly and have been indexed by GoogleBot.