Thero, Bhadanta, and Venerable.. are words for addressing the old monks, abbots in pagodas and/or temples. I really do not know how they are ranked in Buddhist Sangha. Which title is the highest... and what else?
From AccessToInsight's Glossary of Pali and Buddhist Terms:
Literally, a small samaṇa; a novice monk (nun) who observes ten precepts and who is a candidate for admission to the order of bhikkhus (bhikkhunīs).
A Buddhist monk; a man who has given up the householder's life to live a life of heightened virtue (see sīla) in accordance with the Vinaya in general, and the Pātimokkha rules in particular.
"Elder." An honorific title automatically conferred upon a bhikkhu of at least ten years' standing.
"Great elder." An honorific title automatically conferred upon a bhikkhu of at least twenty years' standing.
The female equivalents are samaṇerī, bhikkhuni, theri and mahātheri.
"Venerable" is the English-language translation of Bhadanta (or Bhante).
The word literally means "elder". These terms, appearing at the end of a monastic's given name, are used to distinguish those who have at least 10 years since their upasampada (higher ordination).
See also Ajahn which appears to mean the same thing ("for Buddhist monks who have passed ten vassa") but in Thai language