There are many types, too, and you should never buy until you've figured out which is right for you. Measurements and adjustments are required to ensure the frame is the correct size and that the seat and handlebars are properly positioned. And, every new bicycle will need a minor tune-up after several weeks of use.
What's more, modern road and mountain bikes can travel at high speeds and carry you over demanding terrain where the machine's materials and construction get tested. A quality two-wheeler, carefully assembled and fit to you, will be the safest, most durable and fun bicycle to ride.