Concepts are the elements of propositions rather in the way that words are the elements of sentences. Concepts are abstract, universal mental entities that serve to designate categories or classes of entities, events or relations. Concepts are abstract in that they omit the differences of the things in their extension, treating them as if they were identical. Concepts are universal in that they apply equally to every thing in their extension.
Concepts are bearers of meaning. A single concept can be expressed by any number of languages. The concept DOG can be expressed by 'hund' in German, 'chien' in French, 'perro' in Spanish. The fact that concepts are in some sense language independent makes translation possible; words in various languages "mean the same" because they express one and the same concept.
Theories of concepts: classical; prototype; externalist Concept acquisition v. nativism