My girlfriend and me are looking for bikes to use on the weekends for short trips around town. I think we are supposed to buy "Hybrids". We have a budget of about $150.00 for each bike. I was told to look in Craigslist for used: Cannondale, Trek, Raleigh, Specialized and Scott. So far, I have only seen Mountain Bikes and not Hybrids; I may be wrong since I have trouble discerning the difference.

My question is, where is the best place to buy a used quality bike, for the best price, to use on the weekends? Is there an online depot of quality used bikes? what are the brands that are worth buying? How much should I pay? What is the difference between Hybrid and Mountain Bike; which do I need?

I could really use some help with this purchase. Thanks everyone.