I know someone that has it. Her one side of her face dropped a bit. Leaving her mouth like a sneer. It has improved through time. Here is a site that gives you loads of info.
http://www.bellspalsy.ws/
usually occurs on one side of the face, it is inflammation of the trigeminal nerve, causing numbness and paralysis of the facial muscles.
If caught early, can be treated with steroids to reduce inflammation and hopefully alleviate the symptoms.
About 20% of those that get Bell's Palsy are permanently damaged.