Hopefully some Dutch players themselves can add to this, but to start this thread off, the Dutch is a "system" type opening both in the sense that black can use it against a variety of openings and move orders from white and he can play, more or less the same series of opening moves, regardless of how white opens. Typically these are the moves the Dutch player aims at and roughly the order he will play them in, unless white offers up something like the Staunton Gambit (1. d4 f5 2. e4)
1. d4 f5 2. c4 Nf6 3. Nc3 e6 4. Nf3 Bg5 5. Bd3 O-O 6. e3 d6 7. Bd3 b6 8. O-O Bb7 9. Nd2 Nbd7 10. Re1 Qe8 11. e4 Qg6
White may vary his opening moves quite a bit from that example, but black generally adheres to such a template since the Dutch is based on the idea that black avoids initial engagement, allowing white to develop independently, preferring to defer confrontation to the middle game where he intends to launch a vicious, all out attack against the white K. Thus it is more about general ideas than opening theory, and the more mating patterns and motifs you know the better you'll do with the Dutch. Often the attack is conducted down the g file with support from a N on e4 and/or a B on b7 with one or more of the g and h pawns sometimes acting as bayonets to try and pry open white's castle.
There are three variations that black can adopt: The Stonewall, The Classical, and the Leningrad.
The Stonewall and Classical are fairly similar, the difference being mainly that in the Stonewall black fixes the center with his pawns on d5, e6, and f5, while in the Classical he prefers to keep the center a little more fluid with pawns on d6, e6, and f5. The Leningrad is a fairly independent variation where black adopts a Kside fianchetto where f5 is usually played on the first or second move.