1. What is the difference between peat, compost, and "peat moss"?
Peat is harvested from peat bogs--many of them are located in Canada. Peat moss is the main component in peat, and it decomposes very slowly in the bog due to the wet, anaerobic conditions found there. Compost is different in that it generally contains a wide variety of organic materials that have been allowed to decompose in aerobic (plenty of oxygen) conditions.

2. Do Jiffy "peat pots" and "peat pellets" contain peat, or sphagnum peat moss?
They are made almost totally, if not completely, of peat. I'm not really sure, but there may be something added to get the things to hold together. I used them once...never will again. (Don't freak out if you've already used them for starting seeds--they will work OK, though there are better choices. Watch the watering--don't overwater, but realize those peat pots/pellets can dry out very quickly).

3. Is peat or sphagnum peat a good source of plant nutrition?
No. It contains almost no nutrients. Plants growing in containers have to be fertilized. Use a very weak fertilzer solution on seedlings after they develop true leaves.

4. I have heard that sphagnum peat has a very acidic PH. Does this mean it is bad for growing plants and germinating seeds?
Lime is added to raise the pH in commercial seed starting mixes and potting soils that contain peat or other acidic material. Its usefulness in growing plants is often debated, sometimes fiercly (though not because of the pH thing). I think it makes a very good base for seed starting mixes, and can be useful as a component in container soil. There are usually better choices for amending "in the ground" soil, for which peat is sometimes used.

Jason

Peat moss is a naturally occuring, non renewable resource from boggy areas. Most of it sold in the USA is from the bogs of northern Canada where habitat destruction is occuring as this is harvested. Other than its value as organic matter peat moss is more a detriment to your soil since you need to add large volumes of nutrients to feed the Soil Food Web that would digest this material if it had any food value at all.
Compost is a manufactured product where all the vegetive waste is mixed together and digested by cerain bacteria changing this vegetative waste into a soil amendment that wil feed the Soil Food Web who will feed your plants so they grow strong and healthy.
Peat is just a shortcut name for peat moss.
Sphagnum moss is peat moss that has not been processed as much as the very fine sphagnum peat moss some people add to their soil, same stuff different label. All these names, peat, peat moss, sphagnum peat moss, are all interchangeable.
The peat pots are simply pots made from highly compressed peat moss. The peat pellets are a highly compressed planting media, but the are all made from the same product a non renewable resource.
Peat moss or sphagnum peat moss has no nutrient value and in your soil without some added nutrients will cause plants to exhibit nutrient deficiencies as the soil bacteria try to digest that almost undigestable peat.
All peat mosses are acidic, but do they really change your soils pH? This is presumed, but not true in my experience. No pH change in the soil in a test bed was found 1 year after adding a 3.8 cubic foot bag of peat moss (label said Canadian sphagnum peat moss) into a 4 x 4 bed 8 inches deep.