This article is from the Hanggliding and Paragliding FAQ, by Joao Geada with numerous contributions by others.
The tow vehicle is an ultralight aircraft designed to fly at the
very slow speeds needed to safely tow a hangglider. The towline is
attached at one end to a release on the ultralight and on the other
end to the hangglider. The ultralight flys up to altitude with the
hang glider flying under tow behind, then the hangglider releases.