I, 2, or 3, are the HP ratings.
1 HP: Trim routers are smaller in diameter and usually come with a fixed base, but the newer ones like Dewalt have both a plunge and fixed base.
2 HP: or up and down within 1/4HP, like the Porter cable 690 at 1 3/4 or the PC 895 at 2 1/4 HP, or Bosch, Freud, Makita, Craftsman, almost any brands with both fixed and plunge base IF you get the "kit"
3HP: Like the Porter Cable 7518 with a fixed base only and great for table mounting, Milwaukee 5620, Triition, Makita 3612, etc.
You really want to have one of each because they do different and specific tasks better, however you can start out in the 2 HP class and be OK.
Trim routers with 1 HP, are for small jobs, with 1/4" bits like laminate or edge trimming. The 3HP "big boys" are for spinning larger 2" and up with 1/2" bits for profiling panels and edges. And the 2 HP can do some of each. I know a fellow woodworker who has had a table mounted PC 690 for years and spins 3" panel bits successfully.
A "D" handle makes for easier one handed control, but generally you want both hands on the machine and the work securely clamped down or heavy enough so it won't move around.
Amazon has a pretty good selection:
http://www.amazon.com/s/?ie=UTF8&ke...ools&hvadid=3482153942&ref=pd_sl_6c149l7ei7_p