High Availability and Fault Tolerance have the same objective of keeping your system running in case of component failure / outage.

They are although different in design, costs and behaviour.

An application that is highly available will react to a component failure and quickly recover.

A Fault tolerant one, can instead tolerate any component fault to avoid any side effect like performance impact, data loss, or system crashes.

An architecture can be highly available without being fault tolerant, or can be both.

High availability is achieved by removing single points of failure using system redundancy.

Fault tolerance is achieved by adding even more redundant resources, and at different levels, increasing uptime, but also complexity and costs.

If you have 4 EC2 instances and a Load Balancer, it will direct your traffic to the 4 instances, if one fails for some reason, traffic will be directed to the other 3 ( until, eventually autoscaling launches a 4th one again).

If your instances are in different AZ and the entire AZ fails, you still have 2 other Instances running, so your system is Highly Available ( and to some extend Fault Tolerant to the AZ level), but if the Region fails, your system would have a downtime.

Deploying your system to different regions, makes it Fault tolerant, but you can now see how things get more complex and expensive?

Now, let's start from the beginning and discover AWS EC2 Autoscaling, Elastic Load Balancers and CrossZone Balancing. 

https://www.linkedin.com/pulse/high-availability-vs-fault-tolerance-jon-bonso/