A person will make the call to prayer, the adhaan, to indicate the prayer time has entered. Many worshipers will stand up and perform individual extra prayers called sunnah. Then something called the iqamah will be recited, indicating the congregation for the obligatory prayer is about to start. If people were spread out across the hall before, they will begin lining up shoulder to shoulder, waiting for the imam to say Allahu Akbar (God is Greater) and raise his hands to begin the prayer.
In congregational prayers, the imam is the leader who will recite certain things aloud, and everyone else will follow quietly. The prayer ends when the imam sits and says Assalaamu alaykum wa rahmatullah, turning his head to the right and then the left. After the congregational prayer, the imam will usually say some supplications before people then begin to stand up and either do more extra prayers or make their way to the exit.