These would refer to those processors which were around 5 or more years ago. They will typically come with older refurnished or budget computer systems. Be very careful if you are purchasing such CPUs as your system's speed will definitely slow to a crawl. Worse still, some software on the market today simply cannot be run using older processors.

The Windows XP operating system, for example, needs the newer crops of CPUs to function. Overall, I'd only recommend buying these processors if you want to do extremely simple tasks with your PC - like word processing or general web browsing. Some examples of processors in this category would be: the older AMD Athlon CPUs and Intel Celeron CPUs, AMD Duron CPUs, and Intel Pentium III CPUs.
 

Budget Processors

Strapped for cash? Consider purchasing CPUs in this category - which include yesterday's top-of-the-line CPUs which have been discontinued or the newer budget CPUs. While you may not get extremely good performance, these CPUs will give you the best bang for your buck.

While you're not going to get super-duper performance for more CPU intensive activities like gaming and video editing, the speed provided by these CPUs will be more than enough for general applications. Under this category, you'd be looking at processors like the AMD Athlon XP, AMD Sempron, Intel Celeron (2.4 GHz and higher), as well as the Intel Pentium 4 (2.6 GHz and lower).
 

Mid-Range Processors

The CPUs in this category would have been king of the hill just 6 months ago. Due to the entry of newer, faster CPUs, processors will drop in price and become more affordable. These processors will still scream in performance and are very popular among current PC owners.

Whenever I upgrade my computer, I always go for CPUs in this category. To sum up: thes CPUs here give slightly outdated but extremely powerful performance. Examples include the AMD Athlon XP (2800+ to 3500+) and the Intel Pentium 4 (2.8GHz and higher) CPUs.
 

High-End Processors

Want drop-dead performance? The CPUs in this category will scream even if you run the most demanding gaming or video editing applications in the market today. Of course, be prepared to fork out serious cash. If you absolutely want no less than the best, be an early adopter and go for one of these CPUs.

Personally, I tend to avoid buying top-of-the-line CPUs for two reasons - the high prices and stability. These CPUs are not only expensive, they are also unproven - I've heard cases of extremely new CPUs crashing or overheating. CPUs like the AMD Athlon 64 FX and the Intel Pentium 4 Extreme Edition fall into this category.
 

Conclusion

In summary, always consider your computing needs when selecting a CPU. If you want to do simple word processing or general computer tasks, go for the budget or low end processors. If you are more into gaming or do CPU intensive work like video editing, go for a mid-range or high-end processor.