Google Maps data is based on many sources, depending on the type of data and the area you are looking at.
Vector (data Road, Park, etc.)
Vector data, such as roads, attractions, etc., are bought from different companies. Tele Atlas is one of its global data providers and is a key component, especially outside of densely populated urban areas.
In some areas, this data is combined with other vector data providers, such as Sanborn, which perform 3D contours of the building, and also combined with more local data sources, such as organizations that collect POI data (restaurants, etc.).
In non-US countries, data is often purchased from the National Cartographic Agency; a government agency tasked with collecting and disseminating map data.
In some cases, data, especially to fill out requests, is collected via the Internet and geocoded (viewed by address) for placement on a map.
This data is commercial; aspects of data collection are expensive, and Google pays a significant amount of money to license the data for this use. (The actual amount is not publicly available.)
Images
Image data for Google is similarly collected at many sources. Images up to .5M / px (so you can see cars clearly, but not people) are usually collected via satellites flying from Digital Globe or Geoeye. (Geoeye actually flies to the Geoeye1 satellite, which is heavily funded by Google.)
In addition, Google adds many different sources of information, from state organizations and programs (USGS, NAIP), state and local organizations, etc. In addition, for high-profile events, Google sometimes pays companies specially for flights - this was in the event of an earthquake in Haiti, and it is usually customary for them to do during the Burning Man festival.
Street View
Streetview data is collected on vehicles paid by Google to drive with special tools (LIDAR + 8-way videocameras detectors) and collect data.
In general, in each case, you can look at various data sources - at least those that require lending, and this is not all of them - in the lower right corner of any Google Map.