It is called the UNION PEARSON EXPRESS ( or UP EXPRESS)
its just under $30 each way for an adult. ( a taxi cab is $50 flat each way and then there is the roadway traffic aka time waster)
However, they have a variety of rider options and rates for your travel needs.
UP Express departs from both Union Station and Toronto Pearson International Airport every 15 minutes. Your journey takes just 25 minutes; it's the swiftest and most reliable way to travel.
MAP of UNION Station area
MAP of YYZ location
Here is a link to purchase your Tickets
If you travel quite a bit, perhaps the app would be a need: