There are three different times to pray - morning, evening and night (Shachrit, Minchah and Ma'ariv). Jews pray at these times out of respect to the forefathers of our religion who began the daily prayers: Abraham "invented" Shacharit, Isaac, Minchah, and Jacob, Ma'ariv. How do we know this ? Well, in Isaac's case, we can find in the Bible (Genesis, 24:63) a reference that says "And Isaac went out to meditate in the field at eventide." There are similar passges for Abraham and Jacob.
In the morning and evening prayers, we recite the Shema. We also lay Tefillin.
Chasidic masters said that morning prayer should be completed by the time the Sun came up, because before this event, the day can be set, but afterwards the day has already started on its course.