what is kvm over ip? can someone explain?
Keyboard-Video-Mouse over IP.
It can be implemented in a KVM appliance that has the ability to forward console sessions to a remote computer, usually in a web browser -- you browse to the KVM, click on the server you want, and you have remote control of the console. True console, in that you can interact with the computer while it is booting exactly as if you were in front of the machine.
More recently, some Lights-out-management cards built into enterprise-class computers sometimes come with a KVM-over-IP implementation built in, so you don't need a separate appliance to get true remote control. HP offers the iLO (although usually the KVM functionality is a increased-cost option) and some Sun X series have the iLOM (I am thinking of the X2200-M2, of which I have a dozen or so).
Of the two (appliance or built-in management card) I prefer the latter, because usually they come with remote power controls as well so you can power cycle a computer remotely even if it is totally hung, then interact with the BIOS or RAID cards or whatever on the way up.
Personally I'm getting spoiled -- my attitude is that if I have to go down and touch hardware, something better be physically broken.
To complete David's answer (that is very clear by the way), here is a solution commonly used on x86 servers: IPMI
You can also build your own wireless IPKVM, if you're really keen :)
The other answers are already good, to throw some more systems into the room: Dell uses iDrac (as far as I remember it is just an IPMI clone) and Intel has with vPro/AMT also the possibility to do kvm (from AMT version 6 there is an vnc server that allows you to manage the machine - but beware: from my experience the AMT is (to say it mildly) a PITA.
My experience with the IPMI and Dell system are positive, they do their job and work more or less out of the box.