As an elite Y and B on NW mean automatic upgrades at ticketing if available. Additionally, you get a 50% bonus in elite qualifying miles for flights booked in those classes. Internationally, Y and B are the only flights that you can upgrade with miles, Y for 30K miles roundtrip and B for 60K miles.

Additionally, many Y and B fares (although NOT all) are changeable or refundable to a certain extent.

S and X are full fares on KLM and have similar terms.

Unless you are an elite, need extra EQM, or a flexible ticket, you don't really need to worry about them.