No one candidate is expected to take an outright majority; rather, four candidates are in contention to make it to the second round: far-right nationalist Marine Le Pen, center-right François Fillon, centrist Emmanuel Macron, and far-left populist Jean-Luc Mélenchon. Of these, Le Pen and Macron are generally considered the frontrunners in a close race, and Le Pen — an immigration and war on terror hardliner — is thought to benefit from the fear created by Thursday's terrorist attack in Paris.
France has a prime minister as well as a president; the current PM is a member of the Socialist Party. The prime minister is generally tasked with domestic policy while the president's focus is foreign affairs.
The vote between Sunday's winners will be cast May 7. In the meantime, read The Week's guide to France's populist uprising, including a description of the top four contenders.