PCMark 7 Basic is a free benchmarking tool intended for systems running Windows 7. It tests many aspects of performance, including video, Web speed, and disk access, and turns the results into a "score" which can then be shown to all your friends (if yours is higher than theirs) or used to justify the purchase of a new computer (if not).
Whereas a tool like ENVy Computer Compare will exhaustively catalog your system, PCMark 7 focuses on testing it: It's not what your computer has, it's how well it works. Real-world performance is impacted by many things, such as slow disk access, excessive background processes, or other bottlenecks or errors. PCMark 7 runs a battery of tests, performing each test three times, so that the results are less likely to be distorted by a momentary glitch or random event. PC Mark Advanced has many "views" (packages of tests), such as "Productivity" or "Computation," while the Basic has only the Overall Performance suite. The Advanced edition will allow you to select or deselect a suite, but not the individual tests within the suite.
Once you've got your score, you can go to PCMark.com and see how well you did. There are quite a number of other products in the "Mark" line, and there are many scores to look at, and this can be a problem: Per the FAQ, the scores from different products (such as 3DMark or PCMark Vantage) cannot be directly compared. So the "2346" score from PCMark 7 might be better than the "19876" score someone else posted using 3DMark, or it might be as much worse as it looks, and there's really no way to tell.
PCMark 7 Advanced allows you to export your results as detailed XML files, while PCMark 7 Basic stores your results online. Multiple results from the same machine can be helpful if you want to compare performance over time (maybe it's not your imagination that things are running slower since your cousin turned off your firewall and visited some 'Hot Gaming Cheat Codes' site that filled your PC with malware), or if you've swapped out a video card and want to get a 'before and after' report. Because both the free Basic and $40 Advanced licenses forbid use in a commercial environment, PCMark 7 is not useful for tracking performance of machines in a corporate network. To use PCMark in a business setting, you must purchase the $995 PCMark Professional.
The extra tests, and offline result storage, of PCMark 7 Advanced are nice, but I'm hard pressed to say they're forty dollars worth of nice. Some of the other features of the Advanced version are an advertising-free website and unlimited online storage of results. The advanced version is worthwhile if you're regularly building machines, testing different configurations, or evaluating specific functions of a machine. For most home users, PCMark 7 Basic will give you what you want: a single number expressing the overall power of your system.