Unlocked simply tells you whether or not a product is open to all networks or locked to a specific one. For example, if a product states EE/Virgin/3 then it will be locked to that specific network, if however it says unlocked or any network you are able to put any service provider into that product.

The A and B seen in the product description is a way pf grading the phone and stating what condition it is considered to be in.

A grade is the best grade possible for a phone/table/laptop/camera. This means that it should be immaculate and have all of its original accessories.

B grade is considered used and may have some general wear and tear marks, which will not affect the usage pf the product. It may not have its original accessories but will come with anything that is required to allow use the product properly.