Lysis took care of the basics in the post above, but it's essentially a storm that isn't entirely cold-core (i.e. extratropical) or entirely warm-core (i.e. tropical). Often, these storms develop a shallow warm-core at low-levels, maintaining their cold-core structure aloft. The wind field isn't as expansive as an extratropical system, but not as tight and well-defined (or as strong) as a tropical system. Convection has some organization with colder cloud tops near the center, unlike an extratropical system, but does not consolidate into a core feature nor does it tend to maintain itself over a period of days (instead opting for bursts), unlike a tropical system.

Sometimes they undergo "tropical transition" (the opposite of extratropical transition), where they sit over warm waters for a number of days well-removed from the mid-latitude pattern and gradually acquire tropical characteristics, like Nicole last year and Ana in 2003, but more often than not they do not. Persistent convection and a contracting wind field (which you can judge with QuikSCAT fields) are generally the best signs that something might be occurring; it takes convection and its associated updrafts coupled with inflow at low levels generally found with a surface area of low pressure to get something going, much like with a tropical wave, but it has the added requisite of overcoming a cold core aloft.

Needless to say, we'll have to watch it, but anything that happens is going to be slow to get going and not anything that should threaten land.