A few months have passed since the last major update, and as is tradition, Path of Exile has now once again added a whole bunch of new content and balance changes to play around with. There's numerous new encounters to tackle, a variety of new items and gems to further extend the customization system, as well as a new crafting system that gives you a chance to create some ridiculously powerful items.
The Synthesis update has also brought with it a rebalance and overhaul of a good chunk of the spells, with a focus on making some of the earliest spells just a bit more interesting to use. Perhaps most importantly, at least as far as end-game diversity is concerned, there are now a lot more incentives for hand-casting spells rather than using some of the support gems to do it for you.
You can read a little bit more about the Synthesis update, as well as read the full patch notes, over at the official forums. Unsurprisingly, the patch notes are absolutely ridiculous in size, so if you do decide to read through all of them, I would highly recommend brewing a cup of coffee beforehand.
If you're just looking for the short version, however, allow me to leave you with the recently posted trailer. It obviously doesn't go in-depth about any of the features, but it will give you a bit of an idea of what Synthesis looks like in gameplay terms, as well as what the new lore is all about. Enjoy!