W is the code for AF's Premium Economy tickets. It is a sub-cabin similar to how SQ, NZ, QF, VS and some others offer a true Premium Economy product.
My understanding is that they are not eligible for SkyTeam upgrades because the alliance doesn't really recognize the Premium Economy cabin across all carriers. But since it earns 1.5x MQMs on DL like Y fares and it is a more spacious seating space I'd definitely take it over a DL M fare.