Glitch Canceling:
Many special moves in MK have a number of hits you can do before that move is locked. Moves like the Harpoon and teleport punch have a limit of 4 hits and the moves are essentially turned off. MKII did not really have a feature like this, but they start implimenting time limit usage on moves, which still exists in UMK3. You can cancel a LP, HP, JP, or JK on the frame of contact, into a special move with a hit limit, one hit beyond that limit. In other games this technique is referred to as "Just Frames" meaning essentially you have just one frame in 60 to get this to work.
Example character: Scorpion
Block a teleport punch, counter with aaHPHP, delayed JK, teleport punch, run in, aaHP(glitch cancel on contact frame) - harpoon. You have just broken the rules and done 5 hits before a harpoon.
Other examples of moves with hit limit cancels: Kitana's Fan lift (0) Sub's Freeze (1), Mileena's Roll (2), Reptile's Forceball(3), Ermac's Telekinetic Lift(4). Sindels' scream has very strange properties. The scream itself has no legal limit, if you connect a single aaHP or LP between screams, you can continue doing screams until they are too low to the ground, however the general hit limit is 2. If you juggle 1 HP, scream, 2HPs, scream, you can continue doing single HP, dancing on that last hit limit.
After this, you can glitch cancel her scream multiple times: aaHPHP(limit reached), aaHPHPgc, aaHP gc, etc. This concept in general is a very advanced tactic and you should stay away from it in game until you know what you are doing, but the easiest character to practice this with is Sub-zero, as you will see more gcing with him than anyone.