What does "rooted" mean as it applies to a Cellphone?

Thanks for the explainer Spotmark - to add to this just a bit, Root access allows not only "customization" in general, but it allows a user to change performance settings of the hardware (such as over/underclock the CPU), or to replace the operating system that the phone has installed.  This becomes useful if a newer version of Android has been released (2.2) but the phone ships with 2.1 and no official updates exist (sound familiar Incredible users?).  But beware, Rooting a phone has potentially fatal drawbacks, such as the voided warranty/support from HTC or Verizon.  But worse would be a user attempting to Root a phone and making a mistake in the process - it could destroy (brick) your phone.  And there would be no one to blame but yourself.  Not trying to sound harsh here, just want noobies to understand the risks of pressing ahead.

As a side note, I have a 1.5 year old Google G1 that I recently rooted, only because I knew I would be switching to Verizon in short order.  I have zero experience with software or hardware development and had no prior experience Rooting.  It worked out fine for me, but the instructions were CLEARLY posted and I spent quite a bit of time researching before I did it.  What it all boils down to - if you want to Root you can get some great benefits (longer battery life, for example), but you are completely on your own if you screw it up.  Good luck!

I am curious as to the origin of the word 'rooted', which sounds like the past tense of an action verb.  'Root directory' is a term of art in the computer business, but 'rooted' appears to be a made-up word - is 'rooted' simply shorthand for 'gain access to root directory'?

And 'bricked'? What's up with that? A brick is 8 by 4 by 3 inches and weighs several pounds - hard to see any similarity between a brick and a dead phone.

Speak English, people.