Cell Selection and Reselection

Cell selection refers to the initial registration that a MS will make with a network. This normally only occurs when the phone powers up or when the MS roams from one network to another.
Cell reselection refers to the process of a MS choosing a new cell to monitor once it has already registered and is camped on a cell. It is important to distinguish that selection and reselection are done by the MS itself and not governed by the network. The network would only be responsible for this function when the MS is in a Traffic Channel (TCH). When the MS reselects a new cell it will not inform the network that it has done so unless that new cell is in a new Location Area (LA).
There are many parameters involved in selection and reselection of a new cell. The MS must ensure it is getting the best signal and the network must ensure that the MS does not cause unneeded strain on the network by switching cells when unnecessary or undesired.