skos:Concept

• A complex concept is a concept that combines two or more simpler concepts. For example, human resource management combines the idea of people with their usefulness as resources requiring management. Complex concepts are sometimes expressed with a single word, but more often they are expressed with a multi-word term.
• A concept is a unit of thought. The semantic content of a concept can be re-expressed by a combination of other and different concepts, which may vary from one language or culture to another. Concepts exist in the mind as abstract entities that are independent of the terms used to label them.
• A concept schema is a set of concepts, which may include statements about the semantic relationships between those concepts. Thesauri, classification schemes, subject heading lists, taxonomies, terminologies, glossaries, and other types of controlled vocabularies are examples of concept schemas.

skos:Concept

The central element of SKOS is the concept.

“Concepts exist in the mind as abstract entities that are independent of the terms used to label them.” [SKOS Primer]

To create a Concept you have to create a Uniform Resource Identifier (URI) and assign this URI an rdf:type skos:Concept. This is how it is done in RDF syntax:

@prefix skos: <http://www.w3.org/2004/02/skos/core#> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix ex: <http://www.ejemplo.com/> .
ex:cat rdf:type skos:Concept.

Concepts

The fundamental element of the SKOS vocabulary is the concept. Concepts are the units of thought, as stated-ideas, meanings, or (categories of) objects and events-that underlie many systems of knowledge organization [SKOS-UCR]. As such, concepts exist in the mind as abstract entities that are independent of the terms used to label them.

SKOS introduces the skos:Concept class, which allows implementers to assert that a given resource is a concept. This is done in two steps

creating (or reusing) a Uniform Resource Identifier (URI [URI]) to uniquely identify the concept.
by asserting in RDF, via the rdf:type property, that the resource identified by this URI is of type skos:Concept.
For example:

<http://www.example.com/animals> rdf:type skos:Concept.
This can also be represented in Turtle in a more compact form using the ex namespace prefix defined above:

ex:animals rdf:type skos:Concept.
Using SKOS to publish concept schemas makes it easier to reference concepts in resource descriptions on the semantic web. Implementers are encouraged to use HTTP URIs when coining concept URIs, as they are resolvable into representations that can be accessed using standard web technologies.