rom - What is the purpose of a ramdisk in Android?

I am reading a link that explains what partitions exist in Android.

Can someone tell what is the purpose of ramdisk in Android ?

The link says ramdisk is present (along with the kernel) in /boot partition. However, when I try to do some searching based on that, I encounter pages where people try to copy boot.img and ramdisk.img to the phone.

Is ramdisk part of boot ?

Thanks.