Arduino Mega 2560 R3 PWM Frequency Control

Hi, I have a 4-pin fan that can be controlled with PWM to control its speed. I need it to operate at 25KHz.
Also +2 pins I need them for a motor PWM control with the same frequency, lets say 4KHz.

I need information regarding how to control the Arduino's PWM pins.
From what I googled, there are 16bit timers and 8bit timers. For the mega which one is which?
What timers are responsible for what pins?
And which timer the arduino code uses for delay and millis functions?
From my google's searchings it became complicated real quick. There are two modes for the timer it seems... and there are different prescaler values for each timer. Different equations of course. And then there is a "TOP" variable....

Can somebody help me sort my confusion? :blush:
I'm really tired with all these "secrets" modes and I would be happier if someone could put or redirect me to all the info on the same place. And in orderly manner so I won't be confused. Please. I hope I don't ask a lot.

Just give analogWrite a try it is rare that you need to adjust the advanced stuff with pwm to work good for stuff like that.