how does manual focus work on a lens (Nikon)?

Manual focus is just the same as auto except you've got to do it yourself whilst looking through the view finder.

F-stop (or aperture) will effect how much is in focus in front of or behind the main thing you focused on.

Say you focus on a flower in a field - with a low f number the aperture will be large the exposure time shorter and little will be in focus in front of or behind the one flower (nice effect), with a higher f number the aperture will be small(er), the exposure time will be longer and more of the field will be in focus around the one flower.

The time to switch to manual focus is when you want something sharp and the camera keeps focusing on something else in the picture - which might happen if that flower is off centre in the image composition.

When the lens is wide open (f/1.8 or similar small number) the depth of field is very shallow while when it is closed down tight (f/22 or similar larger number) the depth of field is much deeper - but of course much less light is getting through, so you normally have to use a slower shutter speed.

With manual focus you can open up the aperture and then adjust the focus so only a person or a single blossom is in focus and the background and foreground are blurred, keeping attention on the object/person.

Compare on this page http://mikegigi.com/firthm/victria3.htm the long scenic shots with great depth and the tight bloom shots where the leaves behind are blurred - and various mixes.

Have a look at this webpage. It will really help you to understand: http://camerasim.com/camera-simulator/

Some other tips:

Learn about your camera !!!!

Read your manual !!!! Read it several times, and practice in between. If you don't have a book, download a new one from the manufacturer's website.

Attend a class, visit your local library for some books, read/watch online tutorials.

There are many resources available to us online, some of those provided by the camera manufacturers (and NO, you do NOT need an Olympus camera to learn about general photographic principles just because those hints are on an Olympus websites - Photography is Photography.)

For example:

http://www.olympusamerica.com/cpg_section/oima_lea...

http://www.learn.usa.canon.com/galleries/galleries...

http://photographycourse.net/

Understanding at least the basic principles of photography (as well as being familiar with your camera) will enable you to make the most of your gear, and will make a HUGE difference to your images.

Flick the switch to MF and rotate the lens focus ring until the image in the viewfinder is sharp

BEFORE autostop, every camera had to be manual focused by turning a screw based lens until the photographer likes the effect, sharp for clarity, fuzzy for ...Whatever.

turning the lens by hand is manual focus.

Yes