The TOEFL® is one of the most widely accepted English profeciency language tests in the world. 
This test measures your ability to communicate in English, which is something dental schools here in the States want to see as part of your application. The TOEFL® test is accepted by 7,000+ destinations in more than 130 countries, which is more than any other English language test in the world. Virtually all dental schools in the US will want you to show evidence of a certain minimum 
score on your TOEFL® test (minimum scores vary from one school/program to another).

You can take the Internet-based Test (iBT) or the Paper-based Test (PBT), depending on which format is offered at your test center. Some schools or programs still accept older scores from the Computer-based test. The TOEFL iBT measures how well you read, listen, speak and write in English and use these skills together in the university classroom. For example, you might read a passage or listen to a lecture, and then write or speak about what you learned.

As with any other test, you need to be familiar with all of the regulations and small details of the TOEFL test, the best source is the official bulletin published by ETS:
Download the 2009-10 iBT TOEFL Bulletin (PDF) 
Download the 2009-10 PBT TOEFL Bulletin (PDF)

TOEFL® Test Preparation:
There are endless TOEFL test preparation resources, books, and tools that you can use, here are some of the best ones:

- TOEFL Practice Online: this is the official iBT TOEFL practice test published by ETS, and it uses real iBT TOEFL questions.

- Official TOEFL Channel on You TUBE: Tips from real people to improve English language skills.

- Test Tips: direct from the TOEFL people.