're' is a sambodhan (like we say 'hey'), not so polite. Better u learn by listening its usage by natives. ka,ke,ki these show possesion. aap KA/KE/KI - your kaviya ka/ke/ki - kaviya's or of kaviya ghar ka/ke/ki - home's or of home use of ka ke or ki is decided by the gender of the thing which is under posession or belongs to the subject. e.g. book is feminine so we say aap KI book. (your book) kaviya KI book. (kaviya's book) 'ke' is a bit complicated used when belonging(s) is(are) plural. aapke paise. similar rule u can apply on RA/RE/RI KA/KE/KI resp meRA ghar (my home) MeRI book (my book) friend if male meRA friend female friend meRI friend mere friends (my friends) usage of 'ko' is a large answer in shortest way i can write it: this word used in Accusative/objective and dative cases. look up them i guess u can relate it with ur native language.

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Its not very polite. Teenagers use this a lot, but it is frowned upon. Not professional and you wouldn't want to say it to your parents or anything like that. VERY informal!