The short version
In Southern high-riser culture, a donk is a 1971-76 Chevrolet Impala or Caprice, a box is a 1977-90 Caprice or Impala (the squared-off body), and a bubble is a 1991-96 Caprice or Impala (the rounded body). Lift any of them on very large wheels and you've got a high-riser.
If the '64 Impala is the face of West Coast car culture, the big-body Chevy on huge rims is the face of the South, and Florida is where it got loudest. Same idea, opposite direction: instead of laying the car on the ground, you put it as high in the air as it'll go.
Where the name comes from
The most common story is that "donk" came from the Impala badge. Look at the leaping impala emblem from a certain angle and somebody decided it looked like a donkey. You'll hear other versions too, including a link to the "ba-donk-a-donk" slang, and nobody has a paper trail. What's settled is the usage: in the scene, a donk means the 1971-76 body, not any car on big wheels. Calling a lifted Charger a donk is how you find out who knows.
The look
Wheels. The wheel is the headline. Builds that started on 20s and 22s moved to 24s and 26s, and show cars go much bigger. Fitting them takes a lift, cut fenders or both, and a lot of work on brakes and steering geometry so the car still stops and turns.
Paint and theme. Bright candy and pearl paint, often matched to a sports team, a soda can, a cereal box, or a favorite candy. Interiors get matched too, sometimes down to the trunk lining.
Sound. Like the slab and the scraper, the trunk is usually a showcase for the system. A donk with a quiet stereo isn't really done.
Why big wheels are a real engineering problem
Bigger, heavier wheels change everything: the car's center of gravity, the load on the ball joints and bearings, how far the brakes have to work to stop more rotating mass, and how the speedometer reads. Good builders upgrade brakes, steering parts and sometimes the whole front suspension. Cheap builds skip that, and that's how you get the videos of wheels coming off. If you're buying one, ask who built it and what was changed under the car, not only what's on the outside.
Music and the scene
Southern rap put these cars everywhere. Florida artists from Miami bass on through Trick Daddy, Trina and Rick Ross filled videos with high-risers, and Atlanta and New Orleans scenes had their own versions. The car shows, especially around the big summer weekends, are where the culture really lives: rows of matched paint, people comparing wheel sizes, and systems turned all the way up.
Same root, different branch
High-risers get laughed at by some lowrider purists, and vice versa. Both camps are doing the same thing. They take a common American sedan and turn it into something personal that's meant to be seen and heard on the block. Read about how lowrider clubs handled that, or how Houston did it with slabs.