The London System begins with 1.d4, with an early Bf4, pawns on c3/d4/e3, often with Bd3, Nbd2 and Ngf3. The opening is more about positional development, and flexible piece play, than outright attack; there are solid lines against QGD, KID, QID, Dutch et al. The London System can transpose from/into Colle System (c3) and Stonewall Attack and -- in some lines with c4 (instead of c3) -- into Queen's Gambit territory. The true strength is, with limited theory, one almost always reaches a playable middle-game; the goal of most of us amateurs!
See video: http://youtu.be/-I0t_FD4ZwY