There are not dialects of Spanish language. What you mentioned are other languages. Spanish is an extremely homogeneous widespread language, and part of the Latin languages family.
Castilian is Spanish. The language was called Castilian before Franco dictatorship, who changed the name to Spanish because of his nationalist ideology.
Galician and Catalan are Latin languages, like French, Spanish, Portuguese, Italian and Romanian. All of them have status of languages in the same terms, and they are not dialects at all.
Euskera, or Basque language, is according with Wikipedia: "Linguistically, Basque is unrelated to the other languages of Europe and indeed, as a language isolate, to any other known living language".