Tina - yes, WEMS are longer mountain bike races usually 3, 6 or 12 hour events (and some can be done as teams). It's a loop that varies in distance and difficulty that you ride for the time alloted. While it operates on a point system for the series, there aren't classes or levels based on ability. Here is their website: http://www.wemseries.com/.

WORS are mountain bike sprints and done individually. They also run on a point system but riders are in a class based on ability, much like road races I believe. The event is a shorter loop than WEMS and only 1 or two laps are done (I think - I know they added some distance to some events in the past years). Here's the website: http://www.wors.org/news/.

I have been to both events but only ridden in WEMS races so I may be biased. In my opinion (please don't shoot me if you disagree!) WORS events are much more competitive and aggressive with a lot of testosterone flying around. WEMS are more laid back, family oriented (kind of festival like atmosphere). Both have great athletes and the competition is tough, just very different atmospheres.

The two courses up here are completely different as well. Nine Mile in Rib Mountain (Big Ring Classis WORS, and 24 hr of Nine Mile WEMS) is fast with lots of single track compared to the Underdown in Merrill (Thunderdown in the Underdown WEMS) which was once motorcross trails. They have been working hard out there adding trails so they're softer and much more difficult with steeper hills and swampy moist soil. Again, in my opinion, the Underdown is much more challanging and hard core mtbers tend to prefer it while racers at heart would probably prefer Nine Mile.

Hope that helps.