While researching the three major credit bureaus, many months ago I learned that "hard" inquiries will negatively (e.g., temporarily reduce) affect your credit score. This Consumerist blog post contains the first good definition I've read of what constitutes a "hard" credit inquiry:
"A hard inquiry is when a person or organization requests your credit score and history and they intend to make a lending decision. Applying for a credit card? Hard inquiry. Getting approved for a car loan or mortgage loan? Hard inquiry. On your reports, each of the credit unions categorize these inquiries differently. TransUnion calls them "regular inquiries," Experian calls them "requests viewed by others," and Equifax calls them "Inquiries in the last 12 months." Hard inquiries usually drop your score by a few points for six months, then their effect is removed."
A "soft inquiry" is everything else. I don't worry about "hard" inquiries affecting my credit score because I have removed the causes. First, I opted out of pre-approved credit offers. Second, I placed a Security Freeze on my credit reports at the three national credit bureaus, so nobody can make a credit report inquiry without my consent. The primary reason I placed a Security Freeze on my credit reports was to protect my sensitive personal data since IBM exposed my identity data during its February 2007 data breach.