A panel saw is a specially modified style of circular saw for cutting large sheet goods such as plywood, melamine coated particle board, MDF (medium density fiberboard) or OSB (oriented strand board). Most of these sheet good products are sold in 4-foot by 8-foot sheets (among other sizes), which can be cumbersome to muscle into place to cut on a table saw. Conversely, while cutting sheet goods by hand with a circular saw as they're positioned over a pair of sawhorses isn't nearly as accurate as using a table saw to make the cuts.
Benefits of a Panel Saw
A panel saw can provide the accuracy of a table saw without needing to lift the board and push it through the saw blade. A traditional panel saw requires a lot of space, as it typically is mounted against a wall. The vertically-oriented board is placed onto a series of rollers and is slid into position beneath the saw, which is then cut and pushed downward through the board. For a rip cut, the saw is oriented horizontally, is locked at the proper height above the rollers, and then the plywood is positioned onto the rollers and is pushed through the saw.
This simply isn't a practical setup for most smaller (and home-based) woodworking shops, but it is ideal for cutting sheet goods at a retail establishment. Pay a visit to your local home center in the lumber section, and more often than not, you'll find a panel saw mounted against the end cap of one of the lumber racks. This type of saw is perfect for employees to cut large 4-foot by 8-foot sheets into smaller sizes for their customers because they are simple to operate and when properly set up and tuned, are quite accurate.
Panel saws cut either parallel or perpendicular to the long axis of a piece of plywood. They aren't equipped for making angled cuts, nor typically are they equipped for making beveled cuts on the sheet goods. In this way, a table saw has some definite advantages over a panel saw.
Panel Saw Systems
In an effort to bring the simplicity and accuracy of a panel saw to the small-scale woodworker who may not have the space for a panel saw (or even a table saw), some tool manufacturers have built panel saw systems that allow you to clamp a dedicated circular saw to a flange equipped with roller bearing guide wheels that glide along an aluminum track. The aluminum track is attached to the sheet of plywood to be cut using a pair of small woodworking clamps to hold the system in position, then the saw is pushed along the track, thus cutting the board.
Note that on most of these systems, you could also attach a fixed-base router to the base and use the system to guide your router.
One of the drawbacks to such a system is that mounting the base of the circular saw onto the flange base can be a bit tedious to get it aligned properly to prevent binding when cutting. Once you have the saw mounted to the flange base, you'll likely want to leave it mounted for future use, which means that you won't be able to use the circular saw without the panel kit.
Homemade Panel Saws
A "homemade" option to a dedicated panel saw kit might be to simply use a high-quality extendable metal straight edge as a guide for your circular saw. These straight edges are typically available in four-foot sections but come with an insert that allows you to connect two (or potentially, more) sections end-to-end for cutting standard sheets of plywood along the longer eight-foot axis.
The advantage over such a system is that it doesn't require you to dedicate a circular saw to the system; you simply measure the distance from the edge of the saw blade to the edge of the base of the circular saw, then offset this distance from the desired cut line and attach the straight edge to the plywood with a small C-clamp or other small woodworking clamp (I like to use the Bessey U-Style or L-Style LMU Light-duty Woodworking Clamps with the plastic pads on each edge of the clamp to prevent marring the plywood). With the clamp secured, you merely hold the base of the circular saw against the straight edge as you make the cut.
One of the potential drawbacks of using a straight edge instead of a dedicated panel cutting system is that if you're not extremely careful, the blade's cutting motion can cause the base of the circular saw to stray away from the straight edge, causing an inaccurate cut. With a little bit of practice, I've found this setup to be just as accurate as cutting sheet goods with a table saw.