The central processing unit (CPU) runs the instructions that make programs work. When you open a browser tab, edit a document or calculate a spreadsheet, the CPU performs many small operations on data.
Instructions and data
A program is eventually represented as machine instructions the CPU can understand. An instruction might load a value, compare two values, add numbers or move data. The CPU repeatedly fetches instructions, interprets them and executes them. Modern processors overlap many of these steps, but the basic idea remains useful.
The CPU works closely with RAM. Frequently used data is also kept in tiny, very fast caches inside the processor. If needed information is not in a cache, the CPU waits longer to retrieve it from RAM. Storage is slower again and is used to keep programs and files between sessions.
Cores and threads
A core is a processing unit within a CPU. Multiple cores allow independent work to happen at the same time. A program that can split work into several pieces may finish faster on a CPU with more cores. A simple task that depends on one sequence of instructions may gain less.
Some CPUs expose more logical processors than physical cores through simultaneous multithreading. This helps keep execution resources busy, but two logical processors sharing a core are not equivalent to two complete physical cores.
Clock speed and performance
Clock speed describes how many clock cycles occur each second, usually in gigahertz. It is one useful characteristic, but it does not tell you how much work a CPU completes per cycle. Architecture, cache, power limits, temperature and the software itself all affect performance. Comparing two processors by clock speed alone can therefore be misleading.
What a CPU does not do alone
A CPU does not permanently store your files. It relies on storage for that and on RAM for active working data. Graphics processing may be handled by graphics hardware integrated into the CPU or by a separate graphics card. The operating system schedules work across available processor resources.
If a computer feels slow, first identify the task and the component under pressure. High CPU use can be normal during a demanding job; it is not, by itself, proof that the processor is faulty.