Recently Samsung decided to introduce regional SIM locks for the Galaxy S3, Galaxy Note 2, Galaxy S4, Galaxy S4 mini and Galaxy Note 3. This means you can only use the devices with a SIM card of the region where it was sold. For example: if you purchase a device in Europe you can not use it with a prepaid SIM card outside Europe. It is also not possible any longer to use imported devices in Europe.
After Samsung first said that this lock only applies to the first activation of the device and you can then still use it with SIM cards from other regions this seems not to be true. It is also said that this lock will be applied to older devices as well by a firmware update.