This will probably get moved to intro section of full-sized headphone section, but I'll break it down for you real fast.
Here's a great article to read up on. http://www.innerfidelity.com/content/how-planar-magnetic-headphones-work
Some of the cheapest (recently made) good planar magnetics on the market right now are the Mr. Speakers Mad Dogs, Hifiman HE-400. The Mad Dogs are a mod of the 99 dollar T50rp.
As far as general sound differences between planar magnetics and dynamics, planar magnetics in general seem to have very low extending bass that has very low thd at the same time. They also excel in transient response, and all seem to have a natural, smooth decay to them in contrast to dynamics, which at times can seem unnaturally damped or drastically slow. I think this leads to the general consensus on the planar magnetic midrange sounding liquidy compared to dynamics.