Karen Uhlenbeck was selected by a committee of five mathematicians nominated by the European Mathematical Society and the International Mathematics Union. Her work involves the study of partial differential equations, calculus of variations, gauge theory, topological quantum field theory, and integrable systems. The Abel Prize was established on 1 January 2002 – 200 years after the birth of Niels Henrik Abel. The purpose is to award the Abel Prize for outstanding scientific work in the field of mathematics. The prize amount is 6 million NOK (about 750,000 Euro) and was awarded for the first time on 3 June 2003.
EDIT: This was originally a live post from the announcement ceremony in London, but due to requests for more information on her mathematics I have now written a short post (including the full citation from the announcement) on my website: https://tomrocksmaths.com/2019/03/19/abel-prize-laureate-2019-karen-uhlenbeck/