From StrategyWiki, the free strategy guide and walkthrough wiki

As PCs are made of many interchangeable components, they can run games with differing levels of performance — unlike video game consoles, where the components are fixed, and so a certain level of performance is guaranteed for each game.

The main three system specifications which affect game performance are:

• the CPU frequency (in Hertz), which is the number of operations the processor can do per second;
• the amount of RAM (in bytes), which is the amount of working space available for a game;
• and the amount of video RAM (in bytes), which is the amount of working space available on the graphics card, for storing textures.

Each of these affects game performance in a different way, but if they're below the recommended specifications for a game, that game is unlikely to run well, or even at all in some cases.

Additionally, there are various other parameters which can affect whether you can install or play a game on a computer:

• the amount of disk space required (in bytes) affects whether game can be installed fully, partially, or at all;
• the optical drive speed (units explained here) affects the data transfer rate if the game's running off an optical disc;
• your internet data transfer rate decides whether you can play online or not, and some games require an internet connection;
• your operating system version decides whether a game executable will run on the computer, with games being written and compiled for specific versions of operating systems or their libraries;
• and your DirectX version affects whether a game using DirectX will run on Windows — many games use features only available in certain versions of DirectX or higher.

[edit] Measuring specifications

On StrategyWiki, any game which has its system requirements available will have them displayed in the infobox on the guide's main page, in a section similar to the one on the right here. Each system requirement is shown with a bar displaying how it relates to a modern average gaming PC. In order, the specifications are (with maximum values):

• CPU frequency (max: 3000MHz)
• Amount of RAM (max: 2048MiB)
• Amount of disk space (max: 10240MiB)
• Amount of video RAM (max: 1024MiB)
• Internet data transfer rate (max: 8192Kibps)
• Optical drive speed (max: 52x)
• DirectX version (max: 10)
• Windows version ("98", "Me", "2000", "XP", "XP (SP2)" or "Vista")
• OS X version
• Linux version (generally information about kernel and glibc compatibility)