makrooh

Meaning of makrooh

Q: In several answers you use the word "makrooh", for example in the answer to the question "Omitting a Surah in-between two Surahs". Can you please clarify what you mean with "makrooh". Is it sinful to do a "makrooh" act as a one off? If so, what is the difference between "makrooh" and "haram"?