Trip Alerts allows you to do 2 things:
1) Get email alerts when a price drops on a particular route
For example: All flights from Toronto to Vancouver May 2 - May 10
When to use: When you want to find the lowest price with any airline flying your particular route
2) Get email alerts when a price drops on a specific flight
For example: Air Canada flight 225 from Toronto and flight 226 to Vancouver May 2 - May 10
When to use: When you want to find the lowest price but are set on a specific flight itinerary with a particular airline