Minecraft has changed quite a bit since its early days, and most of those changes have been for the better. Features like smooth lighting and seamless biome transitions have improved the game's visuals dramatically, and dozens of content updates have brought new blocks to build with, new enemies to fight, and new dungeons and cave systems to explore.
For better or worse, though, all of this new content has made Minecraft a considerably heavier game than it once was in terms of performance. Modern gaming PCs will obviously have no trouble running the game, but for those with weaker devices, it can be tough to maintain smooth framerates.
Fortunately, the Microsoft and Mojang teams are always looking to make the popular block-based title as accessible as possible - now, as long as you have a web browser, you'll be able to experience Minecraft; albeit in the form of a significantly-downgraded "Classic" version.
The game will play just like it did in 2009, meaning smooth lighting is out, several old bugs are back, and all of the features and content updates mentioned above are nonexistent. It's Minecraft as you remember it during your teenage (or younger) years.
It's purely a creative experience, so there are no tools to craft, no monsters to fight, and only 32 block types to build with. Regardless, it'll undoubtedly be a fun little diversion if you're having a particularly slow day at work, and you can invite up to 9 friends along with you to revel in the nostalgia.
If you want to try Minecraft Classic's browser port for yourself, you can do so on the official website.