A reader with good comprehension will be able to recall specific information that is presented in a text, such as dates, names and the order in which things occurred.
Comprehension is also about understanding the point of a passage. Consider the introductory paragraph of this article: a reader with good comprehension could not only define reading comprehension and reading fluency, but also explain the difference between them.
Comprehension is tested on standardized tests like the ACT. Testing for fluency is not common, but people who read fluently are viewed positively at school, work and other public situations.
Both comprehension and fluency depend on knowing the words being read, hence developing vocabulary is an important way to improve these two reading skills.
In addition to developing vocabulary, exposure to reading material that is made increasingly difficult over time will improve these both reading skills. Listening to fluent reading by a good reader also fosters fluency development.