What is the difference. Layman terms please. Thanks.

I'm not an expert, but FWIW, when you tighten the spokes, you are putting them under (into?) tension. To refer to tensioning the wheel means to check & tighten all the spokes, not just one.

Tension refers to the spoke trying to pull two objects (hub & rim) together. The opposite would be compression, where two objects (hub & rim) try to push the spoke together (er, squish it :) ).

A properly built wheel will have somewhat equally tensioned spokes.

A wheel may be tensioned differently for different usages as well. A wheel built for long easy rides may not be as tensioned (tight) as one built for a sprinter for instance. At least this is what it seems from reading the mechanics forum lately ;)

What is the difference. Layman terms please. Thanks.

My understanding is that tightening is how you tension a wheel. How much you tighten (or not) determines the tension.

You might post to the mechanics board. The question you might be asking is truing, vs wheel building. There is also specialized tools tensionometer, truing stand, ect.

http://www.parktool.com/repair/readhowto.asp?id=128
http://sheldonbrown.com/wheelbuild.html

The problem is the rim must be centered around the hub, centered between the fork/brakes, and tensioned to the correct amount(about 100 PSI). Getting those 3 correct takes a bit of skill. The spokes on either side or the rear wheel will usually have different tensions as the wheel is not symetric.

Exactly right. In the "perfect wheel" all the spokes are equal tension (usually around 100 in lbs if i remember correctly) with the hub centered between the wheels. On most bikes this is only true on the front because the spokes are equal length on either side. The rear wheel will have higher tensions on the drive side because the spokes are shorter due to the hub being pushed in to make room for the cassette.

So basically in truing a wheel you're correcting out of tension spokes that are pulling your wheel out of round (latterally and radially or side to side and up and down if you prefer). Tensioning the wheel would involve bringing everything back to where it should be. Think of it as part of overhauling a wheel.

What is the difference. Layman terms please. Thanks.

The difference is, if you walk into a bicycle shop, and ask them to tighten the spokes, they will likely ask you, how much? If you ask them to tension the wheel, then they will likely know you want the optimum tension. The proper thing, if your asking a local bike shop (LBS) to work on your bicycle is to ask them to tension and true the wheels, as these two activities always go together.

In many ways the process is like tuning a Violin or a Guitar, there is an optimum amount of tension on a Violin string, just as there is on a bicycle spoke, and it may in fact be different for every spoke on the wheel, to keep the wheel true (completely round, and not moving side to side). Although if there is a huge difference, between the spoke tension on one area of the wheel and another on the same side, then it could be that the rim is out of round. If a spoke is not tense enough, then it is more likely to break, if it's too tense, then it could pull through the rim, destroying the rim. There is some latitude though, each spoke on a wheel could have slightly different tension, but still be tense enough to not break, but no too tense to pull through.

If you get a new wheel, it should be tensioned and trued when new, by the bike shop selling it to you, some will do it as matter of course, some will do it if you ask nicely, and some will charge extra for it. If they say, don't worry about it, ask them point blank if they did it, or not. If they didn't, then ask them to, if they don't want to, then walk out, this should all happen before they see your money. After 100 miles or 160km, the wheel should be checked to make sure it's still true, and proper tension.

A good check, you can do yourself, is to pluck each spoke, it will sing, side by side spokes on the same side of the wheel should sing a similar note, it may not be identical, but should be similar, your checking for spokes that sing quite different notes, also spin the wheel, looking from the side, it should show as round, you can use the side of the brake as a guide, if the rim hops at one point, then your wheel isn't true, second check is looking from the back of the bike, apply the brake until it not quite touches, spin the wheel again, if it touches at one or more points, then it's also not true. The wheel should then be re-tensioned and re-trued.

Thanks to all...I guess I need to head to the LBS and drop off my wheels. I got the set from Mike Garcia and have put about 600 of 700 miles on them and now the rear wheel seems out of true.