What are the differences between "bits and bytes"?
Graphs are in bytes, lines are usually measured in bits. A byte is eight bits. For example, a T1 is 1.54 Megabits divide that by eight to get bytes, and you've got roughly 193 KiloBytes.

Why the bytes?
That's what your browser shows when you download things... Same applies to modems. A 28.8 Kilobit modem equates to about 3600 bytes or 3.6 KiloBytes. All of these measurements are expressed over time, ie: this many bytes can go over the circuit PER second.

Why the two ways of measuring the speed?
Well, the Bytes/sec is easier to express, you wouldn't sit there and say "Wow, I'm downloading at 28,800 kilobits here!", it's easier to say 3.6 K (short for KiloByte generally). But if you are selling a modem or a leased line the bits/sec number is bigger so it sounds more impressive. Who'd want a 193 KiloByte/sec T1 when they could have a 1.54 Megabit T1? ;)