I think the difference is more between permanent installation and seasonal as described earlier. Most of the seasonal type do have the setup where the water enters and exits at the same side of the panel, making a U turn within the panel (both of the other ends are capped). As noted, these are often 20 ft long.

Panels designed for permanent installation are generally shorter (so they fit on a roof, but you CAN get sizes other than 12ft, they range from 8 to about 16, we got 14 footers to maximize coverage). They also are designed to be installed such that the water enters at the bottom, is forced UP through the panels and returns to the pool from the top (no U turn). These panels normally can be linked together to whatever width you desire using the supplied tubing (or in the case of Heliocol, a special fitting). The water enters at one lower corner and exits at the opposite upper corner.

The other big difference, as noted in a number of recent threads, is that most on-ground, seasonal types of solar panels are filled up with water once and *normally* remain filled with water. Most permanent installations that are above pool height are designed as drain back systems, meaning that when they turn off, all of the water in the system drains back to pool level.

I second the thought of going to www.powermat.com, a very helpful site to learn about this stuff.