Project PDA

The country lock only allows the use of the phone with SIM cards that originated in a specific country or group of countries.

A network lock only allows the use of the phone with SIM cards that belong to a specific networks.

The most common lock is the service provider lock (SP-lock). Many different service providers may use the same physical network (e.g. MVNOs). An SP-lock ensures that the handset is only used with SIM cards for the same service provider that marketed the handset. Service providers sometimes substantially subsidize handsets, and locking the handset improves the odds that its use will economically benefit the service provider.

At this time in most countries it is either mandatory or voluntary for the provider to unlock the customer's mobile phone. However, some providers, including the former AT&T Wireless, never unlock handsets, even after a customer has fulfilled their service Other providers, for example T-Mobile and more recently Cingular will unlock phones if the customer has an active account in good standing for at least 90.

Subscriber Identity Module (SIM)

Subscriber Identity Module is a smart card roughly the size of a postage stamp that securely stores the key identifying a mobile phone service subscriber, as well as subscription information, saved telephone numbers, preferences, text messages and other information. The equivalent of a SIM in UMTS is a Universal Subscriber Identity Module (USIM).

There are SIMs available in many storage sizes, the biggest being the 128 KiB sim, while the 64 KiB sim is most prevalent.

The SIM stores network state information such as its current Location Area Identity (LAI). If the handset is turned off and back on again it will take data off the SIM and search for the LAI it was in. This saves time by avoiding having to search the whole list of frequencies that the telephone normally would.
Each SIM is uniquely identified by its ICCID (International Circuit Card ID).

SIM card reader / writer

A SIM card reader / writer (or simply reader) is a device that allows the contents of a SIM card to be edited. It usually plugs into your computer's USB port.