There are 12 candidates total; I wanted to focus on the main ones, from the main parties, rather than those we hear about only at a presidential election, every five years.

The two round voting is the brainchild of De Gaulle (and Michel Debré who wrote the constitution of the fifth Republic), who was obsessed with being accountable directly to the people. Hence the requirement for an absolute majority of votes (50% of the vote + 1) and a second round to ensure that, in case no candidate has reached absolute majority in the first round.

Parliamentary elections, scheduled for June 12 and 19, also do have the absolute majority requirement for the first round, and a few lawmakers get elected then. If no one gets an absolute majority, the candidates having got a number of votes corresponding to, at least, one eighth of the total of registered voters, can move to the second round. A relative majority is enough to get elected in the second round.

Preference based voting is an interesting system, but I'm afraid it might be a trifle too subtle for us, simple minded French :)