What is a boot disk ?

Hello,
I am a newbie in computers,I have a pc with a 1.4 gh AMD processor 256 ddr ram 60 gb hdd runing WINME,This computer was assembled by another person in another place.
What I would like to know what is the work of a boot disk boot disk and what is the difference between a emergency boot disk and Windows boot disk and a startup disk and if I should have these disks at hand and if when should I use these disks.
Please bear with me if I’ve not been clear in explaining, I am really confused.
Thanks
Jose