A cash account allows you to invest up to the total amount of dollars that you currently have deposited. A margin account allows you to trade up to 2x the value of dollars deposited in your account using borrowed money. This allows you to invest more than the dollars you have on deposit (this is commonly referred to as leverage).
A tactical advantage of a margin account is the ability to buy and sell securities prior to any settlement period ending. Generally, margin accounts are utilized by those who have some background in trading stocks on their own.
Currently, iFlip only offers cash accounts but will be offering margin accounts in the near future.