Each expansion adds new features/stuff to the existing Sims 2 game. (installation order is more or less irrelevant in Sims 2)
Uni = goto college, influence points, 2 additional want slots after graduating, sub-hood (college campus), zombies, natural science career, dorms
Nightlife = downtown subhood, chemistry (turn ons/offs), lifetime wants (or was this in uni, can't remember), perm platium (ditto), vampires, Mrs Crumplebottom, cars
Open for Business = ownable business lots, badges (for restocking/cashering/salesmen/flower,toy,robot making), toy, floral, and robot making workstations, elevators, sevros (sim robots), lockable doors
Pets = pets (dogs and cats only, birds and hamsters are simple objects), pets can get jobs, werewolves
Seasons = seasons (summer, winter, fall, spring), snow, rain/lighting/hail, failing leaves, gardens, growable tomatoes, peppers..etc, temperatures, build snowmen, snowball fights, more badges (gardening...), plant sims, ability to put leftovers in fridge.
bon voyage = vacation subhoods (asianish, lumberjackish, and tikish), hotels, beachs, saunas, bigfoot, treasure hunting (digging dirt), more jewelery options
Freetime = hobbies skills, more workbenches (includes working with clay, rebuilding car..etc), more badges, contests
Apartment life = living in apartments (more than one family can live on a lot, each family treated seperately), lot more kids toys (monkey bars..etc), circular staircase, ceilings, witches, ownable helicopter
Off the top of my head thats what they mostly do (give or take i missed something important) The cheaper stuff packs (glamor life, IKEA, family fun...etc) just contain stuff, no new code features. The christmas packs do include a Santa sim and baking cookies, though.