This website is great for creating itineraries across multiple airlines and has excellent filters to ensure you get the best trip possible. The most useful of these are price, desired departure and arrival time and desired length of journey. I have used this service predominately for booking my flights in recent years.
Once you have selected a journey, the website directs you to the carriers website or a third party (for example, CheapOair) that offers the tickets at the published price. These transactions are secure and all tickets are valid. I’ve noticed that when looking to attach your air miles or frequent flyer rewards to a ticket (I use Velocity Frequent Flyer, through Virgin for example), you may have to call the airline or go through the portal for that airline after your purchase. I’ve had no issues with this, it is just an extra step, and very worth it.
(Note: as I’m looking for flights from Vancouver to Belize here in May 2017, I realise that this website has returned more favourable results for Australia/New Zealand travel, in terms of options and prices, than flights from North America)