..noting that the OP has posted in a VA forum, so presumably wants fare classes for VA.
Y = general reference to all Economy fares. Also "whY", for obvious reasons

W, Y+, PE = general reference to all Premium Economy fares.
J, C = general reference to all Business fares.
F = general reference to all First fares.
The letters themselves represent fare buckets - the ones I've mentioned above are generally the most expensive fare bucket in that fare category. Each airline uses different letters for different fares, so it's important to know that the link
williamsf1 posted will give you all of the fares that VA uses, don't assume that it's the same set of letters for QF (or any other airline for that matter).
Welcome to AFF btw