The exit poll is the crucial bit of information available to tell you who came out to vote, what candidate they voted for, and why. Exit polls are conducted as voters leave their polling place, thus the name "exit" polls. For the states that hold caucuses, exit polls are called entrance polls, simply because interviewers talk to voters as they're entering their polling place instead of exiting. Read more on exit polls.
There are no entrance polls currently available for the Republican party in Iowa.