The new 2024 Player's Handbook for Dungeons & Dragons is officially on sale today. While game store retailers and some D&D Beyond subscribers have had access to the new handbook for a while, the 2024 Player's Handbook is now widely available both digitally on D&D Beyond and various VTT services and through large retailers like Amazon and Target.
While the 2024 Player's Handbook isn't the start of a new edition of Dungeons & Dragons (at least, not officially), Wizards of the Coast used the new rulebook to significantly update parts of the current 5th Edition ruleset. As a result, the Player's Handbook contains new character-building rules, new spell rules, and even new rules changing or clarifying various conditions. The changes are relatively significant, although the core gameplay still feels very similar to the 2014 version of D&D 5th Edition.
Also, using the 2024 Player's Handbook does not disqualify players from using past 5th edition material. Provided that there's not a direct equivalent in the new Player's Handbook, players can use older material (such as subclasses or races) in character creation. However, if a player does use the 2024 Player's Handbook to build a character, they are expected to use all relevant 2024 rules in their campaign.