The FAA answers this question here: Weight Class
Weight class is based are assigned by APO130 based on the TFMS observed aircraft codes. There are 6 categories -- (A) Heavy, (B) B757, (C) Large Jet, (D) Large Commuter, (E) Medium, (F) Small.
(A) Heavy: Any aircraft weighing more than 255,000 lb such as the Boeing 747 or the Airbus A340;
(B) B757: Boeing 757 all series;
(C) Large Jet: Large jet aircraft weighing more than 41,000 and up to 255,000 lbs such as the Boeing 737 or the Airbus A320;
(D) Large Commuter: Large non-jet aircraft (such as the Aerospatiale/Alenia ATR-42 and the Saab SF 340), and small regional jets (such as the Bombardier Canadair Regional Jet), weighing more than 41,000 and up to 255,000 lbs;
(E) Medium: Small commuter aircraft including business jets weighing more than 12,500 up to 41,000 lbs such as the Embraer 120 or the Learjet 35; and
(F) Small: Small, single, or twin engine aircraft weighing 12,500 lbs or less such as the Beech 90 or the Cessna Caravan.
Unknown; refers to unspecified equipment.
"Light" Aircraft follows the same definition as "Small" in the above text.
Wake turbulence is no joke - you can feel it if you come in behind a larger aircraft even several minutes after it lands, and often even if you stay above its glide path and land beyond its touchdown point, as you should.
There was a Grumman Tiger landing at my local airport that didn't pay attention, ignored the tower's "caution wake turbulence" warning, and got flipped inverted and subsequently crashed as a result of wake turbulence from a UH-60 military helicopter that had departed several minutes prior. The pilot, very fortunately, walked away but the aircraft was ruined.
Note that even though helicopters are not mentioned in your post - any helicopter leaves persistent rotor wash and you should be extremely cautious and wait several minutes before attempting to takeoff or land anywhere a helicopter has been.