Adjustable / Separated -- These keyboards allow you the option of separating into two halves and adjusting the workplane of the two sections by tenting (raising the center of the keyboard to reduce pronation of the arm) and/or splaying (rotating each section outward to prevent ulnar deviation, or the rotation of the wrist outwards from their natural position).

Combo -- These are best described as keyboards that include a keyboard and mousing solution all in one.  Typically they utilize embedded touchpads or trackballs as the mousing solution and offer space-saving, as well as risk-reducing, ergonomic benefits.  Integrating the mousing device into the keyboard eliminates the normal reaching for the traditional mouse that would otherwise be placed out to the side of the keyboard.

Compact / Slim -- These types of keyboards typically offer all of the same features and functionality of a standard keyboard, but in a more slim or condensed package, thereby reducing the overall footprint or area the keyboard takes up on your desk or keyboard tray.  Often to accomplish this the numeric keypad is embedded into the alphanumeric portion of the keyboard or the physical size of each key is reduced by a small amount.

Fixed Ergonomic -- These keyboards usually offer some sort of split and/or splayed layout, thus providing some ergonomic benefits.  They don't offer any adjustability, however, and utilize a one-size-fits-all mentality.  These are great for users who perhaps need some split/splay of the keyboard to reduce ulnar deviation, but don't need as much customized adjustability.

Numeric Keypads -- Simple plug and play external numeric keypads that can be positioned anywhere to accommodate left- or right-handed users.  These are also great for use with laptops or many of the alternative, adjustable, combo, or compact keyboards if you don't like working with the embedded keypads found on many of these devices.