The hot wire to the fan in a fan/light kit will usually be black and the light wire will be blue. The white wire is usually the neutral which is always connected directly to the source neutral, either at the source or through a splice in the switch box. The white wire may also be used to carry current when necessary.
Here the exhaust fan is controlled by a timer instead of a switch. There should be two hot wires and a ground coming out of the timer casing, splice one of these to the hot source. Spice the second to the black, cable wire running to the fan. Splice the source neutral to the white cable wire and the ground to the ground wires. At the fan splice the wires, matching the colors of each.
This is the updated wiring for this arrangement, with a 2-wire cable added between the fan/light and switches. The white wire is no longer used for hot and the source neutral is run through to the switch box to satisfy the 2011 NEC requirement of a neutral wire in all switch boxes. All other wiring is the same as above.
This wiring diagram illustrates the connections for dual controls, a speed controller for the fan and a dimmer for the lights. The source is at the controllers and the input of each is spliced to the black source wire with a pigtail. From the controllers, 3-wire cable runs to the ceiling outlet box.