GARMIN uses NAVTEQ maps. Navteq was bought by Nokia for 8.1 billion dollars. Nokia's makes its own brand GPS is called Navigon.
TomTom gets their map data from TeleAtlas (same place Google does), not from Navteq. Navteq had approached Garmin when Garmin had made a higher offer than TomTom to purchase TeleAtlas. Navteq made a long term deal to supply their maps to Garmin to get them to cancel the offer for the purchase of TeleAtlas.
Here is a link
http://www.gpsreview.net/navteq-vs-tele-atlas/ to compare the two. They are indeed very similar. I have found errors in each. Here is one graph that compares them a year and a half ago.
