Many light fixtures use either line (120 volts) or low voltage (12 or 24 volts). Make sure to buy dimmer switches with the appropriate voltage. In addition, when choosing a low voltage dimmer make sure to know whether the Lutron dimming light fixture uses an electronic or magnetic transformer. If you install a "magnetic dimmer" to control a light fixture using an electronic transformer then in all likelihood the light will flicker, and no one wants that. If you have a situation where you want one dimmer to control both a line and low voltage light fixture, then choose a dimmer switch, low voltage, that matches the transformer used in the low voltage fixture.
When connecting the dimmer to power, it should always be connected to line voltage (usually 120 volts). That is, when a dimmer is used to control a low voltage transformer it is always connected between the source of the power and the primary side (the line voltage side; the 120V side) of the transformer; it is never connected to the secondary side (the low voltage side; the 12V or 24V) of the transformer.