The HTC One boasts a 2300 mAh Lithium Polymer battery which is fairly capacious, however there are phones out there with large batteries. As with most phones these days, the battery is sealed within the unit and cannot be replaced.
The battery life is about what we would expect for a device of this nature. Under light usage, the battery can last from two to three days, but under more heavy usage it will need recharging every night.