A Baby's sleep pattern is made up of two types of sleep, REM (Rapid Eye Movement) sleep and non-REM sleep.
REM sleep occurs when the baby is in an "active state". You will notice movement under her eyelids and twitches in her body
Non-REM sleep on the other hand moves through stages from light sleep to deep sleep, when her body is relaxed and it is difficult to wake her. Babies generally sleep in sleep cycles of approximately 45 minutes.
During a sleep cycle the baby will move from a REM state of sleep, into a non-REM state and then return to a REM state of sleep.
So in other words, the baby will start by sleeping lightly, then drift into a deep sleep and then come out of the deep sleep again. The baby should ideally then be able to move from one sleep cycle into the next without fully waking.