How many people responded to my survey?

How can I find out how many people responded to my survey and how many just visited it without answering?

Go to Questionnaires. Click the little arrow next to the survey's name. 

Visitors = all people who accessed the survey

Respondents = everyone who responded to at least one question in the survey.

What does 'Submitted' mean in the Excel spreadsheet of my survey's results? Why didn't all respondents submit their responses?

In the Excel sheet of results, the column titled Submitted tells you how many respondents clicked the Done button on the last page of the survey. Because clicking Done or even reaching the last page of the survey is not mandatory for responses to be saved, the number of people who submitted their answers is almost always lower than the number of rows in the Excel sheet.