There are two main methods of roasting; fluid-bed or drum roasting. The majority of roasting occurs with large drum roasters. I use a 12lb fluid-bed roaster.
Imagine a fluid bed roaster as a hot air popcorn maker. Beans are suspended in a cylinder above heating elements. Air is drawn over the elements and through the beans causing the beans to move in a fluid motion. The person roasting has infinite control over temperature and roast speed, resulting in distinct roast characteristics and uniformity.
In a drum roaster green beans are put in a rotating cylinder and heat is applied, either underneath or through a conduit in the centre.
Heat transfer is much faster in a fluid-bed roaster, a critical factor when driving moisture out of the bean (time to first crack). The less time the beans spend prior to this first crack the less acetic and quince acids forms, which can be quite irritating to the stomach.
I have many customers with IBS or stomach issues who are delighted to be able to drink coffee again.