Sixth Edition/Rules changes

The Classic Sixth Edition card set and the Classic Sixth Edition rules were not one and the same. They shared the title and they were released on the same day, but were different products. The new rules were the rules for all of Magic, not just for the core set. Because the rules were leaked on the The Dojo, Wizards of the Coast released a statement on their website concerning the rules changes, to stop further rumors and to assure the player base the changes were all for the best.

The Stack

 * Batches and series were obsoleted. Spells or abilities now went on the stack. A player could then play another spell or ability or pass. If he or she passed, his or her opponent got priority to play spells and abilities. When they both passed in succession, the spell or ability on top of the stack resolved. Then the player whose turn it is (the active player) got priority again. Players didn't have to wait for everything on the stack to resolve before playing another spell.
 * Mana sources were obsoleted. Abilities that add mana to the mana pool wouldn't go on the stack (these were now called mana abilities). The player simply would get the mana immediately. Spells that produce mana, however, such as Dark Ritual, went on the stack like other spells. Mana abilities could be played only when the player had priority or were asked to pay mana.

Countering Spells

 * Interrupts were obsoleted. All interrupts now became instants, which meant that spells could be countered any time before it resolved. Abilities that triggered on a spell being "successfully cast" now triggered on a spell being "played.")

Damage Prevention and Regeneration

 * The damage prevention step was obsoleted. Damage was no longer dealt ("assigned") and then successfully dealt - it was simply dealt. Damage prevention, regeneration, and other spells and abilities that generated replacement effects were now played just like other instants. When such a spell or ability resolved, its effect now created a kind of shield. Damage-prevention effects created shields that prevented the next damage the target would take. Regeneration's effect created a shield that replaced a permanent's next destruction with regeneration. These shields lasted until used up or until the next cleanup step, whichever came first. If an effect prevented a specific amount of damage, it created a shield that hung around until that amount of damage was prevented.
 * If two different effects could each prevent the same damage, the "shielded" player or controller of the "shielded" creature choose which effect got applied. All damage-prevention spells and abilities were now targeted.

Triggered Abilities

 * In the new rules, any ability that began with "when," "whenever," or "at" (as in "At the beginning of your upkeep") became a triggered ability. When a triggered ability's condition is met, the ability automatically goes on the stack. Its controller chooses all targets for it, and when it resolves, makes all other choices for it. If two or more triggered abilities go on the stack at the same time, those controlled by the active player go first. If one player controls two or more, that player chooses their order. Triggered abilities can no longer resolve while another spell or ability is resolving.
 * Phase abilities were all changed to triggered abilities that triggered when the specified phase or step began.

Phases and Steps

 * Each turn now has five phases: beginning, main, combat, second main, and end.
 * The beginning phase was now defined to have three steps: untap, upkeep, and draw. No spells or abilities can be played during the untap step, and abilities that trigger during untap wait until the beginning of the upkeep step to go on the stack. Effects saying to do something at the beginning of the turn were specified to mean at the beginning of upkeep. Upkeep abilities ("During your upkeep, do A") were now triggered abilities.
 * There were now two main phases in every turn. They were separated by combat, which was now its own phase. Phase abilities played at the beginning of or during the main phase were now triggered abilities that triggered at the beginning of the first main phase.
 * The end phase was defined as having two steps: end of turn and cleanup. The end-of-turn step works just like the upkeep step. When it begins, all triggered abilities that start with "At end of turn" go on the stack. When the cleanup step begins, the active player discards down to the maximum hand size (usually seven cards). Then all damage on creatures is removed and effects that last "until end of turn" end. If any abilities trigger during the cleanup step, they go on the stack, and then the active player gets priority to play spells and abilities. If no abilities trigger, no one gets priority. If any spells or abilities resolve during cleanup, the whole step is repeated. Otherwise, the turn ends.

Combat

 * Combat was now its own phase with five steps: beginning of combat, declare attackers, declare blockers, combat damage, and end of combat. Spells and abilities could be played during each of these steps, but only after the step's mandatory parts had been completed.
 * The declare attackers and declare blockers steps were unchanged, but dealing combat damage now worked differently. The active player announces how he or she wants attacking creatures' combat damage to be dealt, then the defending player does the same for blocking creatures. Tapped blockers now deal combat damage just like untapped ones. Because there is no damage-prevention step in Classic, the combat damage isn't dealt immediately. Instead, it goes on the stack. Players may then play spells and abilities as usual. Nothing that happens to the attacking and blocking creatures can affect damage that is on the stack waiting to be dealt. When the stack reaches the combat damage, it is dealt according to the earlier damage announcements, even if one or more of the creatures in combat are no longer in play. (Note that this rule was changed by the M10 rules update in 2009, and only creatures still on the battlefield deal combat damage).
 * The end-of-combat step works like the upkeep and end-of-turn steps. When the step begins, all abilities that trigger on the end of combat go on the stack.

Miscellaneous

 * You now lost the game as soon as you reached 0 life, not at the end of the phase.
 * Artifacts' continuous abilities now worked the same way as other permanents' abilities. They no longer "shut off" while the artifact was tapped.
 * Under Classic rules, you choose modes and targets for a spell or ability (and pay costs) when you play it, but you make all other choices when the spell or ability resolves, not when it's played. You can identify modal spells and abilities by the "Choose one--" phrase.