:welcome:
To answer your questions (to the best of my ability)
Rooting is taking full system control of your device, it allows you to do things that normally only the software manufacturers are able to do
A kernel is kind of like a CPU governer it can control how fast or slow your CPU works and the voltage it consumes to reach these speeds
A bootloader is essentially the initial boot sequence of a device, the Original Droid and the Fascinate have unlocked boot loaders...meaning you can take the original source codes and build your own ROM and boot it up on your device
The Droid 2 and X have locked bootloaders where they can still be Rooted, a custom ROM is a more difficult task to achieve
Hopefully someone else will share a little bit more technical explanation as mine is pretty simple but I don't want to give you false information so I am only sharing what I know
Again Welcome, and you would be making a good choice with the Fascinate