Aus, A clean/clear ESN means that that the device can be activated with the proper provider. A bad ESN phone cannot. It has been flagged as stolen, payment due, lost, and for a variety of other reasons. Those phones may be good to get for parts since they are usually cheaper. There are some sites online, like this one, that let your check the ESN.
If your cell phone is unlocked it can be used with more than one wireless carrier like T Mobile and Cingular. The carrier locking is called by several names including "Sub Lock" or "SPC Lock" or "Sim Lock". This is/was done so that the free phone you received from your carrier with a i.e. 2year commitment could not be used on another carriers service.
Hope this helps, good luck.
Clear/clean ESN :
A clear/clean ESN indicates that it is activated properly by the service provider. An unclear ESN refers to a phone that is either still activated, or lost/stolen. Providers will not activate a phone with a bad or unclear ESN. Stay away from those.
Unlocked phone :
If the phone gets free from SIM lock is known as unlocked phone . SIM lock is also called as subsidy lock/NP lock/Country lock/Network lock.
It indicates that the current Network provider restricts our phone to use with other countries. For example: Consider you have an iPhone 4 which is locked to o2 UK . Unexpectedly, you want to switch over from UK to Australia . At that situation, your phone gets locked to particular network (o2). During that time, you need to unlock it to use your iPhone with Australia or other countries. You can get unlocking service from your Network provider/use factory unlocking method/get Remote unlocking method from any of the third-party service providers like Theunlockspot.com to unlock your iPhone 4.
Once, you gets unlocked from your carrier, you can easily switch over from your network to other GSM networks.