Exit polls are conducted after a person has already voted. Individuals are surveyed and their answers are then tallied by the pollster who conducted the survey. The results of those surveys are then compiled and then presented to the public as to how people voted on Election Day. From these numbers, you can identify the strengths and weaknesses of each candidate’s support and of their respective parties.
Take a look at this example from the 2012 Presidential Election. Where were President Obama’s and Governor Romney’s main areas of support? Why would those groups support their candidate in the way that they did?