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)
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).