Crits do roughly 2x damage (technically slightly less) in standard lv100 gsc. At level 5 iirc its more like 1.5x.
Paralysis - speed cut to 1/4. 1/4 chance for fully. grounds immune to that afflicted by twave.
Poison - 1/8 hp dealt at the end of each of the poisoned pokemons actions (NOT between rounds like in future gens). poison and steel types are immune except to that afflicted by twineedle.
Toxic - starts at 1/16, damage increments by 1/16 after each turn of poison. reverts to normal poison after switching out. poison and steel immune. also note that poison (and other events that happen after actions but before the between round phase) gets cancelled after a poke faints, and psn damage is not applied (nor does toxic counter increment) during turns where you KO a poke (important for bp sweeps!)
burn - attack cut in half. 1/8 damage at the end of each of the burned pokes actions. fire immune except to that afflicted by tri attack.
sleep - 1-6 turns inactivity. chance to wake up & attack during the 2nd thru 7th action (inclusve) taken by the pokemon post-sleep
freeze - 10% chance to thaw between rounds (when leftovers usually apply, so you can switch or be roared in and have a chance to thaw out without needing to attack, but if you are frozen before a round starts, you will not thaw during the round a la waking from sleep). Also being hit by a fire move or using flame wheel or sacred fire auto-thaws. ice immune except to that afflicted by tri attack.
leech seed - 1/8 health sapped after each of the seeded poke's actions (a la poison, so you only get same-turn heal if you are faster). all health sapped goes toward healing your poke (not 50% like other drain moves). grass immune.
confusion - 1-4 attacks where a poke can hit itself. snaps out of confusion during the 2nd-to-5th (inclusive) turn following affliction.
attract - 50% chance of being unable to attack. lasts until one poke switches out. same gender and genderless immune.
wrap - 2-5 turns where you cannot switch and receive 1/16 damage (equal to leftovers healng). released on 3rd thru 6th turn (inclusive) after wrap starts.
sandstorm - 1/8 hp damage per turn for non rock steel or ground types. 25% heal from synthesis and the like.
sun - 1.5x fire, 0.5x water, solarbeam 1 turn, thunder 50% accurate, 100% heal from synthesis and the like
rain - 1.5x water, 0.5x fire, half damage solarbeam, 100% accurate thunder (ignores all accuracy checks, its basically swift), 25% heal from synthesis
crits - if atk stage > def stage (ie, you are +1 and they are +0), nothing gets ignored, normal damage is just (almost) doubled
if atk stage <= def stage, ignore everything (all stat changes get set to 0/0, reflect, light screen, burn ignored, but i dont think weather gets ignored)
rollover - normally stats are capped at 999 but the capping code can fail to execute if an item that applies a stat multiplier boosts a stat beyond 1023 (ie thick club at +2 gets marowak to 1032, which rolls over to a whopping 8 attack, light ball and metal powder can do the same) OR if reflect or light screen is up and multiplies def/spdef beyond 1023 (ie reflect up with a +1 def skarmory brings its def down to laughable levels - important to keep in mind against last poke curselax!)
encore - 3-6 turns where you repeat the last move. paralysis or any other turn where a poke is disallowed a move doesnt resey the last move used ID. fails if last move used is sleep talk.
mean look - ghosts are not immune sorry not sorry
counter - fails on moves called by sleep talk, can bounce back all hidden powers (technically normal), even if they run off SpA, cant touch ghosts
mirror coat - fails against all hidden powers, cant touch darks, also fails on moves used by sleep talk
priority - all negative priority moves fail if they are not used after the opponent makes a move (so whirlwind fails if a slower counter/whirlwind/roar/vital throw is used). iirc there is no priority lower than -1 or higher than +4 (protect/detect are +2, switches +3, pursuit on the switch +4 technically)
theres plenty more im prolly forgetting. i find upokecenter (google it, the site is kinda sketch but w/e) is the best source when it comes to specific mechanics info.
also this:
http://www.smogon.com/forums/threads/gsc-mechanics.3542417/