What is a manual focus lens?

Sounds like a stupid question but spare a thought for the person who's just starting out in their photography life and really does want to know.
I wish there had been a resource like the internet for such info when I was starting out. All I could do was take a trip to the local public library and sift through their (not always extensive) books on the subject. 

A manual focus lens is a lens that has a rotating ring fitted with a rubber grip that you twist. The twisting motion moves a helicoid thread within the lens body which pushes or pulls the glass lens elements.
The results you'll see in the cameras viewfinder will be a blurry image until the correct, sharp image appears - the lens is now in focus.

The great advantage of a manual focus lens is the ability to focus on a particular part of any given scene.
For example - you have a countryside scene in front of you but you really want to show the stone footbridge to the left and positioned in the middle ground of the scene.
By focussing on the bridge and setting your cameras aperture to it's maximum setting f2.8 f2 or maybe f1.4, you can keep the bridge sharp and the wide aperture will blur anything in front of and behind the bridge. This is know as selective focussing.

There is also another type of lens and that is the Auto Focus lens.
These will measure the distance between your camera and the subject by using ultrasonic sound waves or infrared light.
The lens (or camera in some instances) has a built in motor that then moves the internal lens elements until the electronics confirm the subject is in focus.

Manual focus lenses have a greater scope for creativity in my opinion and older, vintage ones are often very well made.