Best Answer - Chosen by Voting
"Dom" has for a long time been the spoken language version of "De". Basically you should use "De" when writing formal (a paper for school for example), using "Dom" instead is taboo.
In everyday situations like chatting on IM or conversations you should stick to "Dom", saying "De" just comes out unnatural.
You might also come in contact of "Dem" which is the accusative case of "De". You can read up on that on wikipedia if you are unsure what it means:
http://en.wikipedia.org/wiki/Accusative_case (Dem, used for objects)
http://en.wikipedia.org/wiki/Nominative_case (De, used for subjects)
Know about "Dem" and "De" but use "Dom" (unless you have to write anything formal, then you might want to get someone with Swedish as their mother tongue to correct).
Feel free to PM me if you are have any further questions.
