What Is An Unlocked Cell Phone?
Simply put, an unlocked cell phone is a GSM cell phone that is NOT locked by the manufacturer or network provider to be used in a specific country or or a specific carrier!  You are free to use the phone where you want, and with which ever carrier/network provider that is compatible with the Unlocked GSM Cell Phone.

A Locked Cell Phone is one that has a SIM lock.
A SIM lock, simlock, network lock or subsidy lock — not to be confused with PIN code — is a capability built-in to GSM phones by cellular phone manufacturers. Network providers use this capability to restrict the use of these phones to specific countries and network providers. Currently, phones can be locked to accept only SIM cards from one or more of the following:

    * Countries (the phone will work in one country, but not another)
    * Network/Service providers (e.g. AT&T Mobility, T-Mobile, Vodafone, etc.)
    * SIM types (i.e. only specific SIM cards can be used with the phone).

You can purchase Unlocked mobile phones to work with any GSM provider, as long as it is compatible with your chosen network, but the phone may still display the original branding and may not support all of the features your new carrier (such as Friends & Families type features).

Also Note: That some Unlocked phones may have Network Carrier's Logos imprinted on them.  Such as “AT&T” or “T-mobile”. This in no way inhibits you from being able to use them on any compatible network, or with any compatible SIM card, as these phones are unlocked.