A sintered base is made by compressing polyethylene pellets under high heat, which creates a porous structure that absorbs wax. This makes it faster and more durable than an extruded base — you'll really notice the difference on flat traverses and cat tracks where momentum matters.
Sintered is the industry standard for performance and comes on most mid-to-high-end boards. It's ideal if you ride all-mountain or freeride and want speed and durability against abrasion from rocks and ice. It also holds wax longer than extruded bases, so when properly maintained, it stays fast for more runs.
The catch: you need to wax it regularly. If you buy a sintered board and never wax it, it can actually end up slower than a freshly waxed extruded base. So if you're not willing to commit to tuning every few rides, it may not be the right choice. But if you'll keep up with maintenance, sintered delivers the best all-around performance.