SVC would get very pissed at me if I quoted Chapter K verbatum, so I'll just describe them in brief.
In a nutshell, they are short range Size class 5 ships (Frigates and destroyers are 4, Cruisers are 3, and Dreadnaughts are 2, btw), that usually operate in packs of 6 and are supported by a PF tender (which is usually a converted Light Cruiser) that provides repair, reloads, etc...
They also operate on virtually all bases in the Alpha Octant but the Federation's, (who never deployed PFs). Their Direct fire weapons have about half the range of the bigger ships. Their movement cost in SFB is 1/5 which means thata PF can travel 5 hexes for 1 point of warp.
Flotillas most often operate with one Leader model (slightly more powerful unit), on Scout model (with slightly shorter ranged scout channels) and four standard PFs.
Like the rest of the ships out there, PFs had several variants including All drone, All phasers, Cargo, Fighter Conveyors (Fi-Cons think mini carriers).
They can die easily, but a smart player can mangle, if not destory a few ships before then

I'm wise enough to know that I don't know half as much as I think I know, you know?