The Obama-Biden ticket got a bounce in the polls after the Democratic convention and the McCain-Palin ticket got a bigger bounce after the Republican convention. But the latest national polls indicate a virtual tie in the presidential race. Today's Washington Post poll shows Barack Obama holding 47% of the vote to John McCain’s 46%. What do the polls tell us about how the race has changed since the conventions? Why do all the polls come up with different results? How do the poll results influence voters? What polls do the candidates use in deciding how to deploy their resources and chart the course to victory in the Electoral College?