Sets released: Urza's Legacy, Sixth Edition, Urza's Destiny, Portal:
   Three Kingdoms, Starter, Mercadian Masques

Major stuff:

   - Sixth edition was a pretty complete rethink of the whole timing system
   and card templating, ending up with the 'Grand Unified Timing System'
   - No more batches, interrupt windows, damage prevention bubbles,
   pre-series, phase abilities or phase costs.
   - No more interrupt or mana source cards, these can be played as
   instant cards under the new system.
   - The stack was introduced.
   - The term 'fast effect' was dropped from the rules.
   - The term 'pseudospell' was dropped from the rules.
   - The concept of setting up a prevention or replacement shield was
   introduced.
   - The term 'bury' was dropped from Magic vocabulary.
   - The term 'successfully cast' was dropped as well.
   - The ability 'haste' was introduced and the term 'summoning sickness'
   was dropped.
   - No more summon cards, replaced with creature cards instead.
   - Creature cards with multiple-word creature types now count as each
   separate creature type individually, with many older types being
   hyphenated.
   - No more 'fizzle', replaced with 'countered on resolution'.
   - No more 'cast', replaced with 'play'.
   - No more 'total mana cost', replaced with 'converted mana cost'.
   - Triggered abilities never resolve at the pauses during a spell,
   they all use the stack and resolve independently.
   - Players lose from having 0 life almost immediately, rather than
   waiting until the end of the phase.
   - Combat became a phase of its own.
   - Combat damage uses the stack.
   - Protection no longer absorb all trample damage, as the damage
   assignment ignores the protection ability.
   - Tapped blockers now deal combat damage, they previously didn't.
   - Tapped artifacts don't automatically turn off; Howling Mine, Static
   Orb and Winter Orb got errata to preserve their turning off behaviour.
   - The end of turn step / until end of turn wear-off distinction is
   introduced.
   - The Oracle was introduced.
   - The core set has its own expansion symbol as of Sixth Edition.

   - Urza's Legacy introduced premium (foil) cards.
   - Urza's Legacy also marked a trend for effects that tap or untap
   something to change from only being "target creature, artifact or land"
   to "target permanent"

Trivia:

   - No cards with trample were printed in Sixth Edition.