In the KM literature, knowledge is most commonly categorized as either explicit or tacit (that which is in people's heads). This characterization is however rather too simple, but a more important point, and a criticism, is that it is misleading. A much more nuanced and useful characterization is to describe knowledge as explicit, implicit, and tacit.
Explicit: information or knowledge that is set out in tangible form. (learn more)
Implicit: information or knowledge that is not set out in tangible form but could be made explicit. (learn more)
Tacit: information or knowledge that one would have extreme difficulty operationally setting out in tangible form. (learn more)
The classic example in the KM literature of true "tacit" knowledge is Nonaka and Takeuchi's example of the kinesthetic knowledge that was necessary to design and engineer a home bread maker, knowledge that could only be gained or transferred by having engineers work alongside bread makers and learn the motions and the "feel" necessary to knead bread dough (Nonaka & Takeuchi, 1995).
The danger of the explicit-tacit dichotomy is that by describing knowledge with only two categories, i.e., explicit, that which is set out in tangible form, and tacit, that which is within people, is that it then becomes easy to think overly simplistically in terms of explicit knowledge, which calls for "collecting" KM methodologies, and tacit knowledge, which calls for "connecting" KM methodologies, and to overlook the fact that, in many cases, what may be needed is to convert implicit tacit knowledge to explicit knowledge, for example the after action reports and debriefings described below. Within business and KM, two types of knowledge are usually defined, namely explicit and tacit knowledge.
The former refers to codified knowledge, such as that found in documents, while the latter refers to non codified and often personal/experience-based knowledge. KM and organisational learning theory almost always take root in the interaction and relationship between these two types of knowledge. Tacit and explicit knowledge should be seen as a spectrum rather than as definitive points.
Therefore in practice, all knowledge is a mixture of tacit and explicit elements rather than being one or the other. However, in order to understand knowledge, it is important to define these theoretical opposites.