Your body continually loses water: in urine & faeces, through your skin and from lungs(where you breath out). This means you need to top up levels on an regular basis and drink enough to replace what's lost.
Without perspiration,the normal daily turnover of water in adults is about 4% of body weight, which is 2.8 lit for a 70kg person.
The average sedantary man should consume atleast 2.9L of water a day and the average sedantary women atleast 2.2L per day.
About 1/3rd of this will come from food. In terms of how much you should actually drink,the dietitions recommends the average adult drinks atleast eight glasses (2L) of fluid per day. More during hot weather and physical activity.