Polite birthday greeting. The prefix お adds politeness; the base is 誕生日おめでとう. Used in both spoken and written contexts.
Happy birthday! I hope you have a wonderful year.
Happy birthday to you today. (very polite)
お誕生日おめでとうございます is the polite, teineigo form used with acquaintances or in formal settings, while お誕生日おめでとう is the casual version used with close friends and family.
Phrase composed of polite prefix お + 誕生日 (birthday) + おめでとう (congratulations). The word 誕生日 is a Sino-Japanese compound (誕生 'birth' + 日 'day'), and おめでとう is the polite form of めでたい (auspicious, happy).