I bought a dimmer that supports ic, halogen, led and cfl lights. It says 600w for incadescent and halogen and 150w for led and cfl. Why is the difference? What happens if I put it in a circuit with 8 led lights rated at 25w?
It's not a switch, for one thing. It's a dimmer - a switch, in the same circumstances, would be just fine with full rated amperage of LED or CFL. Unless you have a great deal of interest in dimming, your best option if you have 200W of LEDs on the circuit would be to use a switch.
The limit comes from the interaction of the power supply circuits internal to "dimmable" LEDs/CFLs and the dimmers that can be used with them. Overloading the dimmer will probably cost you at least a new dimmer, and depending what fails, perhaps one or more of the attached lights.