Magnification.
The Xs refer to how much bigger or closer the image looks. If the object is 100 yards away, it will look 10 yards away through a 10x optic.
Drhorsepower is right, some manufacturers make different zoom eyepieces for their spotters. Swarovski makes a 20-60 zoom and a 25-50 WA zoom. The 20-60 has both a lower magnification and higher magnification than the WA zoom. The WA, however, shows a much wider field of view. Think of it as the difference between looking through a porthole (the 20-60 at 60x), and a window (the 25-50). I think wide-field eyepieces are really usefull for hunting. The more of the hillside you can see, the better. That, and the difference between 50x and 60x really isn't that much in practical use.
"The higher the better" sounds good, but if that were the case, we would all have 100x fixed eyepieces on our spotters. I like high-magnification eyepieces as well, as long as they have a nice wide low magnification as well. Those 75x magnifications (The Nikon ED82 and new Diascopes have eyepieces available that go that high) aren't useable very often. First, the exit pupil, with those magnifications, is barely over 1mm, which is pretty tiny. In low light, this makes for dark images without detail. Second, the image isn't the only thing that gets magnified. Atmospheric disturbances, like mirage, also get magnified, making those high-mag images dancing blobs of color.
The standard 15-45 and 20-60 magnification ranges for 65mm and 80mm spotters respectively didn't happen by accident. They provide a nice balance of magnification, field of view, and useable exit pupils. The new WA eyepieces give up very little in terms of magnification, but add large amounts of field of view, which I think is a real benefit. the WA eyepieces are often more expensive, due to their more complicated design.
The new Zeiss 20-75 eyepiece keeps a low, wide field bottom end at 20x, but also offers a very high 75x magnification at the top end, which is nice to use for detail viewing when there is enough light, and low enough atmospheric disturbance.
The spotter with the highest performing image in a hunting-friendly package is the Kowa Prominar 88. It has a variety of eyepieces available, but is usually used with a 20-60 zoom.
mav_7mm,
If you give us a budget and primary uses (range, backpacking, hunting from a vehicle, etc.) we can give you some specific recommendations.