There are several dimensions by which knowledge can be classified into types:

*declarative, episodic, procedural**commonsense, scientific, mathematical*

There is always a relationship between the form in which knowledge is *represented* and the way in which the knowledge is *used*. The study in knowledge representation has been influenced by several disciplines:

- In AI history, the mainstream representation approach is based in first-order predicate logic, though various attempts have been made to modify it to satisfy the needs of AI. According to this approach, knowledge is represented by a set of propositions and rules.
- In some mathematical models, knowledge is represented as points and trajectories in multi-dimensional spaces.
- In some computational models, knowledge is represented as procedures and data structures.

The relationships among different representations are important. For example, here are some tips on translating from English into first-order predicate logic.

- Semantic network: a graph structure in which nodes (or vertices) represent concepts, while the arcs between these nodes represent relations among concepts.
- Frame: similar to object-oriented representation, a frame corresponds to a concept, containing attribute-value pairs (sometimes with types and default values).
- Script: a script represent generalized episodes, or typical sequence of events corresponding to a concept that describes a type of event.

- example 1 (search "Prolog code" in the page)
- example 2 (search "knowledge expressed in the semantic network" in the page)

Description Logic is an attempt to combine some of the above ideas with first-order predicate logic by balancing expressing power and inference efficiency. It is more expressive than propositional logic but has more efficient decision problems than first-order predicate logic. It can also describe concept hierarchies.

Representative knowledge bases in AI:

- CYC: constructing a foundation of basic "common sense" knowledge -- a semantic substratum of terms, rules, and relations -- that will enable a variety of knowledge-intensive products and services. The language is based on first-order predicate logic.
- Open Mind: an attempt of building a huge commonsense knowledge-base in English, participated by volunteers over the world.
- WordNet: an online lexical reference system whose design is inspired by current psycholinguistic theories of human lexical memory.
- Mizar: the project has built a database of mathematical knowledge, which includes more than 2 thousands of definitions of mathematical concepts and more than 30 thousands of theorems.

- Semantic Web: the idea of having data on the web defined and linked in a way, that it can be used by machines - not just for display purposes, but for using it in various applications.
- Ontology: a catalog of the types of things that are assumed to exist in a domain of interest from the perspective of a person.