There are two main types of ventilation fan:
AC FANS are an older technology, cheaper to buy but less efficient and not designed to be speed controlled. AC fans can create hum when they are slowed down by budget controllers, however high end controllers such as the GHC Multifan do offer some hum reduction. Usually heavier and less powerful than EC fans.
EC FANS are newer technology. More efficient and digitally controlled, many EC fans can be speed controlled as standard. Usually quieter as well as less heavy and more powerful. Can also be used easily with thermostatic controllers such as the GHC Multifan and EC fans such as the Hyper Fan even come with a manual speed adjuster included.