From store.apple.com, "learn more" under "processor" where you get to pick:
"Get amazing performance from the dual-core Intel Core i5 or i7 processor, which features two processor cores on a single chip. Choose the speed and processor you want.
The Intel Core i5 and i7 processors are based on new 32-nanometre process technology with an advanced Core micro-architecture that features an integrated memory controller and level 3 cache, giving the Mac mini faster, more direct access to memory. The Core i5 processor has 3MB of level 3 cache; the Core i7 has 4MB of level 3 cache. In addition, these processors feature:
Turbo Boost 2.0 a dynamic performance technology that automatically boosts the processor clock speed based on workload, giving you extra processing power when you need it.
Hyper-Threading a technology that allows two threads to run simultaneously on each core. So a dual-core Mac mini has four virtual cores, all of which are recognised by OS X. This enables the processor to deliver faster performance by spreading tasks more evenly across a greater number of cores."