I am not a native korean but I think I can help :)
sunbae = senior
hoobae = junior
in general : sunbae means people who are on upper grade than you and it is a word that refers to people with more experience (at work, school, etc),
ex: if you are kpop fan so you know that super junior debuted in 2005
and mblaq debuted in 2009 .
super junior debuted before so mblaq memebers will call suju members "sunbae" +nim = more polite ... the same as hyung and hyunmin
and ofcourse it's not only about kpop