Really confused !!!

Hi. Good day to you all.

I am new to the field of computers [and so English (:] and I am having some
difficulty figuring out the way speed is measured in a computer and any form
of sending digital data from one end to the other. I am talking about
internet speed , transferring data from or to an external hard drive or
flash, CD ROM reading and writing, RAM reading, and so on.
So , what is the standard unit for measuring speed in and around a computer?
Is it:

- bit per second, like kilobit per second (Kbps) or Megabit per second (Mbps)
- byte per second, like kilobyte per second (KBps) or Megabit per second
(MBps)
- or is it measured by hertz, like megahertz (MH) or Giga hertz (GH)

What is the difference between the three types?
When are they used?
Are they convertible?
I have read some articles about the issue but it got even more confusing.
Thank you very much in advance for taking your time.

M1