'Trebuchet MS' or Trebuchet MS? Why?


In exercise 4 the instructions tell me to use the font Palatino. That same exercise doesn't tell me what the second 'Palatino Linotype' even does. They also don't explain why Palatino is written Palatino and Palatino Linotype is written 'Palatino Linotype'.

font-family: Palatino, 'Palatino Linotype', serif;

Exercise 6 now tells me that the second font is used as a fallback, in case the first font isn't available. Why was that not explained in exercise 4? Also, why is ' now used the other way round?

font-family: 'Trebuchet MS', Helvetica, sans-serif;