Declarative and nondeclarative memory differ in that declarative memory refers to the recollection of facts and events while nondeclarative memory, also called procedural memory, refers to the ability to perform learned skills or activities. Declarative memory can be expressed or "declared" in terms of information while nondeclarative memory cannot. Declarative and nondeclarative memory are both very important parts of one's long-term memory, as one tends to need to make use of a variety of different facts and skills during any given day. A deficiency or disorder in either form of memory can severely inhibit one's ability to perform one's job or to function normally in day-to-day life.
There are two primary types of declarative memory, referred to as "episodic memory" and "semantic memory." Episodic memory is concerned with the events in one's life and is, accordingly, closely linked to time. One's episodic memory tends to include at least a rough timeline of the events in one's personal history. Semantic memory, on the other hand, refers to the recollection of particular facts and pieces of information and does not tend to involve any particular timeline. A fact tends not to be affected by when it is learned, and most people forget where they learned most of the things that they know about the world.
In contrast to declarative memory, nondeclarative memory is based in the recollection of how to conduct certain actions. While both declarative and nondeclarative memory do involve a form of recollection, the "memories" associated with the procedures contained in nondeclarative memory cannot be expressed in words. Nondeclarative memory involves training oneself in a particular action until it is completely or nearly automatic. In general, one needs to put little or no thought into conducting an action completely committed to procedural memory. Actions such as walking, riding a bike, or typing on a keyboard, which seem completely automatic to many people, are based in one's nondeclarative memory.
Another of the major differences between declarative and nondeclarative memory lies in the ability to refine and improve skills over time. Procedural memory does not stop with learning how to do a particular action. Practice over time can make one more skilled and more effective at conducting that action. Facts and information, on the other hand, cannot be improved through regular use. One can add more information or correct faulty information, but there is no way to make the facts and information in one's declarative memory somehow "better" or more effective.