Thumbtack is essentially a marketplace for time. When you buy a service, you’re buying someone else’s time and the associated skill and effort that comes with it. People purchase services to free up some of their own time (childcare, house cleaning, etc.) and to get the benefit of expertise they don’t have (dance lessons, interior decorators, caterers, etc.). Thumbtack’s goal is to reduce the friction of these interactions.
The founder of Thumbtack, Jonathan Swanson, put it like this:

We’re aiming to create a marketplace where buyers and sellers can easily find and transact with each other. To the consumer, we offer a vetted network and ease of scheduling and paying. To the provider, we offer the support of a back office and the marketing power of having a virtual storefront that we’re bringing consumers to. Thumbtack makes money from each transaction, and from the added services we offer to providers.
Most of the services purchased today are sourced, vetted and transacted offline, but this is changing. We’ve got a big vision–we want to be the infrastructure for the local service sector as it moves online.

Work on Thumbtack began in 2008 and they launched at the end of 2009. Since Thumbtack is creating a marketplace they have two different customers — service professionals, and consumers looking for services. They spent 2010 focusing on signing up service professionals and based on results on their site, they seem to have had a positive response. Tens of thousands of professionals have already signed up all around the country.

I see Thumbtack is what the Yellowbook would have been if it was made today.