I'm not surprised at the question, but he is an extremely important part of United States Space Flight History. I had the honor of knowing the man because he was on my performance review committee.
John Houbolt was the engineer who came up with the idea of having the spacecraft going to the moon to be a 3-part assembly, a Command Module, a Service Module, and a Lunar Module. The Command Module was the portion we think of which, on return from the moon, reentered the Earth's atmosphere and splashed down in the ocean. The Service Module was the portion that provided the propellant and rockets for leaving Earth orbit, trajectory to the Moon, insertion into Lunar Orbit, return to Earth, and reinsertion into Earth orbit. The Lunar Module was the portion that separated from the other two modules, after Lunar orbit was established, and landed on the Moon's surface. Actually it consisted of two parts, one which acted as a launch pad and left on the lunar surface when the 2 astronauts rejoined the Command and Service Modules. (Originally the Lunar Module was called the Lunar Excursion Module, LEM. But since it did not move once landing the name was shortened. When I arrived at NASA as a fresh-hired, junior engineer/scientist it was stilled called the LEM, and that is how I will always remember it.
I came to know Dr. Houbolt (John as he was called by his equals) because when I arrived at Langley Research Center, LaRC, I was assigned to the Guidance and Control Branch of the Space Mechanics Division, where I had worked the prior Summer as a Summer Student. G&C was responsible for the full-size simulation of the Gemini and Apollo Capsule maneuvering in space called the Rendezvous and Docking Simulator, click HERE. This was the group that studied and developed the core components, rendezvous and docking, of John Houbolt's idea into reality. For some of the detail, click HERE. (SMD and G&C was led by some of the best of NASA: Hewitt Phillips, Art Vogeley, Max Kurbjun, Lindsay Lina, and Roy Brissenden.)
John Hoibolt was a member of the committee of senior engineers/scientists who followed, reviewed, and graded my performance for the first year I was at LaRC, June, 1963- June, 1964. As a junior engineer I admired, enjoyed, and clicked my heels, as I did for all the other men listed above. He gave me a high recommendation at the end of that year. I have always hoped I earned it, then and in the remainder of the years I worked in NASA