Notice that the Google Advanced Search generates the tags for your search string: "Site:.org" or "Site:.gov". It can save you from having to remember those tags in the future. A typical search engine will also allow for Boolean and keyword phrasing. Learn more about that here.
You can also copy and paste the url of the website you want Google to find results for (such as if wanting to search for National Geographic or another site for specific articles on a particular topic). Here is an example for an online digital magazine called Undark and the results (notice the results are all from their site):