Celexa is citalopram, also known as Lexapro, an SSRI used to treat depression. It is not recreational and I wouldn't take it if I was prescribed.
Diazepam (Valium) is completely different, it is a weak benzodiazepine that some people find recreational alone or more commonly in combination with low doses of alcohol or other drugs. It is prescribed for treatment of anxiety and insomnia and may be used with SSRIs to deter some of the side effects.