This is a question to ask on the "International Dentist" forum. There are many Indian dental school grads on there trying to get licensed in the US.
Also, this document from the ADA appears to answer all the questions you are asking.
http://www.ada.org/prof/prac/licensure/us.pdf
From my understanding, there are 3 ways to get a license in the US if you are not a US/Candian grad:
1) Do 2 - 3 years in an International Dentist Program (IDP) where you will get the DDS/DMD at the end. Now you are eligible to take any licensing exams you want for whichever state you want to practice in. Most foreign dental grads take this route.
2) Do a residency/specialty and then be eligible to practice in only a few states that will allow international dentists who did US specialties to practice. This is more difficult, especially since many hospital based residencies can not accepts foreign dental grads as residents, so that rules out most of Oral Surgery & Pedo spots. University based residencies can usually accept foreign grads, but programs like ortho and endo are already mad competitive among US grads, so the likelihood of a forein grad getting in is much less. This is addressed in that ADA document.
3) Take the California or Minnesota Bench Test and if you pass, you can get a license to only practice in California or Minnesota.
Seriously, for dental, it is not like medicine. I know in medicine, as long as a foreign grad passes the USMLE, he/she can apply for american residencies. You can't do it in dentistry. You have to do well on the National Dental Board Exam (meaning do much better than just pass it) and then go for one of the three options listed above. Also, there are many licensing exams (NERB, WREB, SRTA, CRDTS, several state specific exams) because each state has different licensing exam requirements, although all states require you to pass the National Boards part 1 & 2.
Also, if you are talking to foreign dental grads who set up shop 20+ years ago, the rules were much different then. The ADA/other organizations have made it more difficult for foreign dentists to get licensed here because the training apparently varies a lot among foreign schools and they want practitioners in the US to have a similar background for competency purposes.