Districts started in 2009 in Michigan. Basically, for the registration fee ($5000 veterans, $6500 rookies) you get 2 events within the district. These are smaller than most regionals at about 40 teams (or smaller). At your first 2 district events, you accrue points (through rank, advancement in elims, awards, etc) to qualify for the district championship (or States when the district is a single state). Then from the district championship you can qualify for St. Louis. So, you get like 24 matches guaranteed instead of possibly only 8 or 10.
2007... bumpers are now required, and show your alliance color (no flags or lights). Robots still weigh 120 lbs, the frame perimeter dimensions have changed a couple times, Championship grew to 600 teams and is in St. Louis, not Atlanta.
We changed from the IFI control system to NI's cRIO and now NI's roboRIO.
FIRST changed their logo a bit and revamped the website (now at
http://www.firstinspires.org).
In 2017, Championship will be in St. Louis AND Houston, and in 2018 it will be in Houston and Detroit. Australia and Mexico finally got regionals, most of the east coast is in districts, 1114 won Chairman's (first non-US team to do that).