The ISO value is a measurement the recording medium's (film, CCD sensor, CMOS sensor, etc) sensitivity to light. The higher the ISO value, the more sensitive to light it is. Standard full stop ISO values are 25, 50, 100, 200, 400, 800, and 1600, but you will also find some in between. ISO 100 is twice as sensitive to light as ISO 50, so if you kept the aperture the same you could cut the shutter speed in half. As you mentioned, increasing the ISO results in more "noise" on a digicam sensor, and using higher ISO film in regular cameras results in loss of sharpness.
So why not always shoot at the lowest ISO you can? Because when the available light starts to diminish, you have to keep the shutter open longer (again assuming the aperture stays the same) to get enough light onto the sensor (or film) to properly expose the picture. After a certain point you can no longer hold the camera still enough to get a good clear picture. You'll probably find 1/60 is as slow as you can shoot (without a tripod), or 1/125 when you're using the zoom. If you were at ISO 50 and shooting at 1/30 then you could up the ISO to 100, and shoot at 1/60. Or ISO 200 and 1/125.
Shutter speed is pretty self explanatory. Standard shutter speeds are 8/1, 4/1, 2/1, 1/1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/500, 1/1000, and 1/2000.
As Auburn9698 mentioned, the aperture is how much light the lens lets through. Standard full stop aperture values are f/45, f/22, f/16, f/11, f/8, f/5.6, f/4, f/2.8, f/2, f/1.4, and f/1. f/1 lets in the most light and f/45 lets in the least. Although there doesn't seem to be much of a pattern, for each full increase in f/stops, you halve the amount of light going through the lens. So if you were shooting at ISO 50, f/8, and needed a 1/30 shutter speed, instead of increasing the ISO as in the previous example you could drop the aperture to f/5.6 and shoot at 1/60 while keeping the ISO at 50 (and sensor noise to a minimum).
So why not always shoot at the lowest f/stop? Two reasons. First is sharpness. Unless you have a really good lens (ie:$2500 lens) you'll probably find that if you back off a few stops your pictures will be sharper. Second is depth of feild, as Auburn9698 mentioned. The higher the f/stop value, the more in focus the things behind of the subject you are focusing on will appear. If you were on a family trip to the grand canyon and were taking apicture of your kids in front of the canyon, you'd probably want to shoot at f/8 to get as much of the background in focus as possible. On the other hand, if you were taking a protrait of your kid and wanted to keep the background distractions to a minimum you'd probably want to open the lens up to f/3.2.
You could probably get more of your plant picture in focus by closing down the lens to f/8.
The f/stop is really a ratio. The A70 has a 5.4 to 16.2mm lens (equivalent to a 35-105mm in 35mm format). When rectacted to 5.4mm (not zoomed) it has the following available apertures:
5.4 divided by f/stop = aperture diameter in mm (aperture area in mm2)
5.4/8.0 = 0.7mm (1.54mm2) - full f/stop
5.4/7.1 = 0.8mm (2.01mm2)
5.4/6.3 = 0.9mm (2.55mm2)
5.4/5.6 = 1.0mm (3.14mm2) - full f/stop
5.4/5.0 = 1.1mm (3.80mm2)
5.4/4.5 = 1.2mm (4.52mm2)
5.4/4.0 = 1.4mm (6.15mm2) - full f/stop
5.4/3.5 = 1.5mm (7.07mm2)
5.4/3.2 = 1.7mm (9.08mm2)
5.4/2.8 = 1.9mm (11.34mm2) - full f/stop
You can see from this table why lower f/stop values let more light in. You can also see that each full f/stop lets in twice as much light as the previous (it's a little off because I didn't carry as many decimal places as I should have).
The manual focus was probably only meant to be used when there isn't enough light to allow the camera to autofocus (ie: night shots).
*Dual PIII-800 @900 i440BX and Tualeron 1.2 @1.7 i815*
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