My computer repair company services thousands of PCs each year. In my experience, there are some constants: No one’s computer or Internet speed ever is fast enough, and all hard drives eventually will fail.

Hard drives physically fail more than any other internal component. That’s no surprise, given that the hard drive is the workhorse of your PC or laptop. This data-storage and data-retrieval device consists of one or more “hard” discs that spin fast and a read/write spindle that accesses and records data on the disc. I liken it to a record player.

Now ponder that the ability of your system to boot up, run programs and give you access to your precious data is resting on this somewhat tenuous structure.

As with every internal component in a computer, manufacturers have developed a series of acronyms and codes to describe everything from size to speed to capacity. Here’s a breakdown of the specifications involved so that when the day comes to replace your drive, you’ll be prepared:

Form Factor: This is the physical dimension of the hard drive. The most popular size is 3.5 inches; that’s the standard size for desktop (tower) PCs. Laptop hard drives typically are 2.5 inches. Hard drives that physically are larger accommodate more storage space, while the smaller drives consume less power and produce less heat — key factors for notebook computers.

Capacity: Anyone familiar with computer-speak probably has heard of megabytes and gigabytes, possibly even terabytes, but it’s hard to guess how much space you’ll need. Documents and device drivers don’t take up much space. You can store 150-200 high-quality photos in 1 GB. A standard-definition DVD takes 2 to 5 GB.

The main factor is whether you take a lot of high-definition videos; the higher the quality of the image, the larger the file. HD content can eat up hard drive space quickly and varies with the length of videos.

If you’re a “gamer,” you’ll probably need lots of space for the data-heavy files that make big games run smoothly. I typically tell the average user not to upgrade capacity beyond the standard 500-GB drive likely to come installed in a new PC, but it depends on your photo and video habits.

RPM: Just as with your car, this means revolutions per minute. As you’d expect, the faster your hard drive’s RPM, the less time it takes your computer to access stored data. Higher RPM may sound nice, especially when you’ve dealt with slower computers in the past, but keep in mind that it comes with a price. Higher RPM can equate to louder machines and generate more heat and thus less energy efficiency. Computers with higher RPM also can fail more often, given that they’re operating in a higher-stress environment. The average desktop user should be comfortable with a 7,200-RPM hard drive, though speeds in the industry can go as high as 15,000 RPM. Laptop users will find 5,400 RPM to be typical.

Cache: Cache is a small area of faster memory within the hard drive used as a buffer between the CPU, memory (RAM) and hard drive. Think of it like a carry-on bag — the bigger it is, the more stuff you can get to quickly without having to dig into your suitcase. You’ll typically find cache sizes of 2 MB, 8 MB and 16 MB in the standard 2.5-inch and 3.5-inch drives. Larger cache size equates to faster performance and tends to cost more.

Stay tuned next week for the lowdown on the next generation of hard drives — solid state drives and hybrid drives — and whether they’re worth the upgrade.