Adding something to Prix here are my summarized comments..
MS operates in two modes: idle mode and dedicated mode. In the idle mode, MS monitors the broadcast channels in order to “hear” if it is being paged. It also measures other BTSs’ BCCH carrier and decides whether it should camp on another cell. This is called cell reselection and the reselection algorithm used in GSM is detailed in [Reference: 3GPP TS 05.08: 6 Idle mode tasks]. In dedicated mode (i.e. during a call), changing cell is called a handover (HO).
Cell selection is performed immediately after MS is switched on. If MS is located in the same cell it in which it was previously was switched off, the SIM card should have the local BCCH frequency stored in memory and MS should find network quite expeditiously. If MS has moved to another cell since it was turned off, it enters a cell selection procedure