In a nutshell, GMRS radios operate over much larger areas and thus require a license for usage. The license cost is $80, is good for 5 years and covers everyone in your immediate family. When you receive your license you will be assigned a call sign and should announce that call sign anytime you begin your initial transmission and then again every 15 minutes if it is a continuous conversation.
Most of the hybrid GMRS/FRS units on the market today require a GMRS license available at
www.fcc.gov . Just because you use an "FRS frequency or channel" on the unit does not exempt you from needing to obtain the license. The radio is type accepted as GMRS and thus requirese the license.
In truth, most folks that buy the hybrid radios don't buy and don't care about the license and are thus breaking the law. Chances of them being caught......

....very little considering the sheer number of these types of radios out there and their limited range. But if somebody causes enough trouble with one then......
Hope this helps some. Check out
www.popularwireless.com for more info.