When you purchase a mobile phone you typically have to sign up for a plan. There are two types of services that your mobile phone will most likely need: voice and data. Voice plans come in many different varieties including Unlimited. Typically a voice plan that states it is unlimited is actually truly unlimited and therefore does not pose any problem.
Data plans are a different story. Data plans are used for accessing the internet from your mobile phone. Uses for these may include checking email, viewing email attachments, viewing online video, streaming music to your phone and even downloading games. As technology advances smartphones become increasingly capable and the uses for data plans on cellphones increase.
Almost all major cellphone providers offer "unlimited" data plans. These so-called unlimited plans are typically not what they appear to be and may deceive the consumer (you). Reading the word unlimited would give you the impression that the plan does not have any type of limitations and can be used as much as you want for whatever purpose you want. In actuality, these plans are limited in the amount of data that can be transferred and the type of usage as well.
The major US cell phone carriers currently include AT&T, Sprint, Verizon and T-Mobile. Some smaller "carriers" have began to appear which rely on the bigger networks to provide service such as StraightTalk and Boost Mobile. These hidden restrictions and limitations are mostly only a problem in the US Market. Throughout much of Europe, especially in countries such as Sweden, flat rate and truly unlimited data plans are the norm for all large mobile carriers.