Buying a computer means making decisions about the pieces and parts that make up a complete set of specifications for a desktop or portable machine. The form factor -- desktop or laptop -- serves as the first choice you make, depending on whether you need a machine to use at home or on the road. After you make that decision, look at five essential parts of the system to refine your overall choice.

Operating System

Choosing from OSX, Windows or an alternative operating system you install yourself guides the hardware selection you make. You can run Microsoft Windows on a Mac in Apple's Boot Camp or a range of alternative virtual machines. As of February 2014, Windows comes in two flavors, one set up for use with a traditional keyboard and pointing device, and one designed for touch-screen use. If you buy a computer without an OS or build one yourself, you can choose among a range of Unix-based options, including versions of Linux.

Processor Specifications

The two main processor manufacturers, Intel and AMD, make various types of chips for use in low-power entry-level systems, mid-range computers and high-end powerhouses. Chip speed, expressed in gigahertz, provides the most familiar overall measure of processor performance. You'll also see specs for number of cores -- physical and virtual -- and amount of cache. The higher the number of physical cores, the faster the computer performs specific types of tasks. If you edit high-definition video or play computing-intensive games, you may need a quad-core or even a six-core processor. Virtual cores become important when you use applications with processes that take advantage of hyperthreading. For basic computing needs, a lower-end dual-core CPU provides ample capability. Cache memory provides data support between storage media and CPU. Computers with higher numbers of cores include more cache.

Memory Capabilities

Look for a system that includes enough RAM to match the specifications recommended for the OS and software you typically run, not just the minimum amount, and enough that you can run more than one application at a time. If you plan to keep using this computer for several years, verify the total amount of RAM it can hold -- number of chips and maximum capacity per chip -- as well as any limitations on how you must install and upgrade memory. Some systems require matched-size pairs of chips, for example, which can limit your options if you want to add more RAM. Memory chips also come in specific speeds and types, further conditioning your upgrade options.

Video Support

Some lower-end computers rely on built-in video subsystems. Rather than a separate graphics processing unit that plugs in to an expansion slot on the motherboard, these designs replace a graphics card with integrated video hardware that you can't upgrade or replace. Video cards can contain multiple gigabytes of RAM to support their own operations and can require specific amounts of electrical power that place advanced demands on the computer's power supply. You'll find the greatest reliance on built-in video in portable and in all-in-one computers, which lack the expansion slots that accept upgradeable video cards.

Storage Capacity

Increasing numbers of computers are skipping traditional platter-based hard drives in favor of solid state drives, or SSDs, which lack moving parts. Based on flash memory similar to the chips in removable thumb drives, these storage devices can provide much faster performance than mechanisms based on moving parts but at a price that still exceeds the cost of standard drives. Once a mainstay of computer components, optical drives -- CD, DVD and Blu-ray readers and writers -- have become less prevalent as stock built-in features of retail computers. If the system you choose doesn't offer one of these drives and your workflow requires it, look for an external peripheral you can plug in to a USB port.