Answer: There is a fairly broad definition of what makes up a "lawn". In my experience, the absolutely "perfect" lawn is a lot of work! If you are truly intent on having a weed-free lawn quickly, and your lawn is currently predominently weeds, then you will probably need to kill the existing vegetation and start over from scratch. Even then, there's no guarantee that weeds won't reappear.
The other option is to accept a slightly less than perfect lawn and achieve a more lush lawn over time. You will encourage the lawn grasses and discourage the weeds through proper mowing (mow higher and more often) and fertilizing, combined with soil improvement based on the results of soil tests. You might also need to reseed in some areas where the lawn is the worst.
The best defense against crabgrass is healthy, dense turf. Since crabgrass is an annual, it doesn't overwinter, but the seeds do. So keep your lawn in peak form with proper mowing, feeding, and watering, and in the spring, apply a corn-gluten-based "weed-n-feed" product. The corn gluten contains an enzyme that kills germinating seeds. If you can prevent the early plants from getting established, your lawn can compete better early in the season and thicken up before the later crabgrass seeds sprout.
You can learn more about the basics of good lawn maintenance (and establishing new lawns) in Lawn Care for Dummies by Lance Walheim. Your county extension office should be able to help you with soil tests and interpreting the results, and they also may have some localized lawn care information available.
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