Looking for the difference between Speed and Bandwidth

Hi, I’m a freshman in college wanting to go into networking. I’m having a little trouble grasping the concept of Speed vs. Bandwidth.

I understand that, say, a Gigabit internet plan has a speed of 1000 megabits per second, or 1000/8 megabytes per second.

Does this, also, then mean that the maximum amount of bandwidth is 1000/8 MB/s? Is there any reason to use anything other than a CAT5e ethernet cable, other than with switches?

Is there even a difference between the two? In my mind, it doesn’t quite make logical sense.