Sleeping bags designed for car camping, which means they tend to be a bit roomier, bulkier, and heavier than those for backpacking (and also, generally cheaper).  Bags rated for X degrees will keep most people comfortable on a night down to that degree rating, but not below- assuming you are sleeping on a sleeping pad with R value 4 or higher.  

How R Value works:

• R value is a measure of how warm a sleeping pad is/how well it insulates you from the ground.
• Higher R = warmer, lower R = colder. 
• 4R is the standard used to set all sleeping bag temperature ratings (e.g. a "20 degree bag" that will keep you comfortable at 20 degrees or warmer)
• If sleeping on a pad with less than 4R (or the ground, or an air mattress, both of which are effectively R=0), that "20 degree bag" is going to leave you feeling very cold on a 20 degree night.
• If sleeping on a pad with with higher than 4R, your sleeping bag will be slightly warmer than