Google has over a decade of searcher data that tells it which types of queries are related to which types of products and services. Therefore, the categories in which you place your company in its Google Places listing are critical to being found for the types of terms people typically use when searching for your kind of business.
For example, when someone searches for chapter 7 lawyer, Google knows that they are looking for a bankruptcy attorney. If you have chosen the Bankruptcy Attorney category in Google Places, then your business is among those eligible to earn a spot in the Local Pack of organic results.
Just because you find one category that describes you well, such as HVAC Contractor, don't stop looking for others. An HVAC contractor might also fall into one or more of these available categories, as well: Air Conditioning Contractor, Air Conditioning Repair Service, Furnace Store, Furnace Parts Supplier, Furnace Repair Service, Heating Contractor, Mechanical Contractor and Air Duct Cleaning Service.
The first category field you fill out is what will be considered your primary category – the one that best describes your business overall. For the business above that would be HVAC Contractor. You must choose from Google's preset categories for this field. Make certain it is the most applicable one for your company.
Also be aware that Google Places has different preset categories for different countries. For example in the UK, you'll find a that Holiday Apartment Rental is a category that does not exist in Places for the US or Canada and that Vacation Home Rental is not an option in the UK.
LocalU's own Mike Blumenthal offers a free Google Places Category Tool that you may find helpful in choosing the right categories for your business.
The categories in which your business is placed in at the major data providers and on other trusted websites most likely has some influence in Google's local ranking algorithms, as well. So be sure to check your categories for accuracy in Infogroup, Acxiom, Superpages, CitySearch and similar places.
Update: There are still some Places accounts in which you can add custom categories. However Joy Hawkins, a Top Contributor to the Places help forum checked with a contact at Google today and learned that they are NOT available in the new dashboard and that any we see are likely a bug. Thanks, Joy!