How can I use grep to find a word inside a folder?

In Windows, I would have done a search for finding a word inside a folder. Similarly, I want to know if a specific word occurs inside a directory containing many sub-directories and files. My searches for grep syntax shows I must specify the filename, i.e. grep string filename.

Now, I do not know the filename, so what do I do? A friend suggested to do grep -nr string, but I don't know what this means and I got no results with it (there is no response until I issue a Ctrl + C).

grep -nr 'yourString*' .

The dot at the end searches the current directory. Meaning for each parameter:

-n            Show relative line number in the file
'yourString*' String for search, followed by a wildcard character
-r            Recursively search subdirectories listed
.             Directory for search (current directory)

grep -nr 'MobileAppSer*' . (Would find MobileAppServlet.java or MobileAppServlet.class or MobileAppServlet.txt; 'MobileAppASer*.*' is another way to do the same thing.)

To check more parameters use man grep command.