pedal vibrations

Ok, i'm not very good with cycling terminology. But I noticed on my mountain bike yesterday, after riding through a trail that after descending I noticed my pedals were vibrating while stationary. I thought it may have been a fluke, but it continued to vibrate whenever I was descending down hills. What is the problem???This never happened before and I thought it could have been my rear wheel being bent but it was not???

If you can grab the pedal with your hand & wiggle it, then the pedal needs to be serviced, ASAP, or replaced ...

Not all pedals are readily serviced ...

The pedal body rides on bearings which sleeve over the pedal's spindle ... on the ends, there is often a nut & cone OR equivalent (cartridge bearings) ... you don't want the nut to actually become loose because I think some pedals could subsequently slide off their spindle!

thanks Alfeng, this sounds like very good advice...I will check the pedals ASAP....its just the basic flat pedals and if it is the pedals, I was going to upgrade to clipless pedals from crank brothers...