To focus, you put your finger on the shutter button and slightly press it, just half-way. When you do that, the camera automatically focuses on the subject. To actually take a picture, you continue pressing that same button all the way down, until it clicks :)
Point-and-shoot cameras allow only autofocusing (just like described above), while SLRs support both manual and autofocus. To focus manually, simply turn the focus ring on the lens, until the subject is sharp (according to your eye), and then shoot. The manual focus ring on the lens can be located either closer to the camera or at the end of the lens, near the glass, depending on the lens type.
Before late 70-s, the only focusing mode available on cameras was the manual focus. Approximately at that time the autofocus was invented, and Nikon released their first camera with autofocus feature in 1983. However, just like with everything "auto", those who considered themselves professionals didn't view that feature useful, but after some time adopted it anyways. Think of autofocus vs manual focusing just like an automatic transmission vs manual in cars. True car enthusiasts drive only manual, while others prefer automatic for comfort. In cameras, autofocus became very precise, so that distinction is minimal or non-existent. Traditionally, Nikon's cameras are flawless when it comes to autofocus, focusing exactly where you want them to, while Canon has autofocus problems even now in their professional $3000 camera models.
Because in SLR cameras the focusing is a feature of the lens, new lenses had to be created to support autofocus (with special contacts between the lens and the camera, so they both could communicate). Nikon labels all their autofocus lenses with an "AF" prefix. There had to be a motor that would turn the focusing ring on the lens (internally, without the ring on the outside actually moving). However, there was a choice as to where to place that motor: in the lens itself, in the camera body, or both. Nikon chose both: there are certain camera models and certain lens models with the autofocus motor inside. When it comes to current Nikon cameras, here's which cameras have an autofocusing motor inside the camera body:
Beginner: No (D40, D60, D3000, D5000)
Advanced: Yes (D90, D300, D300S)
Pro: Yes (D700, D3, D3X)
It means that for the cheapest camera models that don't have an autofocusing motor built-in, the autofocusing motor must be inside the lens, otherwise it will not be possible to autofocus at all. Therefore, in addition to "AF" lenses, Nikon created "AF-S" lenses with an autofocusing motor inside the lens. Now even those cameras that don't have a built-in focusing motor could autofocus, because the lens had that motor inside. By the way, some people wrongly assume that the letter "S" in "AF-S" stands for "speed" or "silent" or "self" or other words they come up with. Even though the lens is fast and silent, the "S" stands for "servo", the motor.
Even though the autofocus required the lens redesign, Nikon kept the lens mount the same on new lenses (the lens mount is the metal ring on the lens which attaches to the camera body).
Nikon is one of the only two manufacturers (the other being Pentax) who kept the lens mount the same, allowing the photographers to use older lenses or lenses that they already had, with newer cameras. Nikon's lens mount has a name, it's called an "F-mount". Canon abandoned their lens mount design when switching to autofocus cameras and lenses in late 80-s, effectively making their customers lose all the lens investment they made so far, and start from the scratch. Canon also chose to place their autofocus motor in all their lenses, therefore any Canon SLR camera (even the cheapest models) can autofocus with all lenses (unlike Nikon). However, with Nikon's beginner dSLRs and a lens without a motor, while you can't autofocus, there's a helpful in-camera guide that will tell you when the subject is in focus. Just press the shutter release button half way and start manually turning the focus ring on the lens. While doing so, look at the bottom left corner of the viewfinder until you see a green dot light up and stay lit.
That means that your subject is in focus now and you can shoot. I find it very handy with my AF 50mm lens, and with little practice I will soon master the manual focus as well. :)
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