Target Tracking: you can choose a metric and whenever the metrics reported by CloudWatch match the conditions (below or above certain value) a new instance is launched.

AWS recommends scaling on metrics with a 1 min frequency.

Simple Scaling: an Alarm is attached to a scaling group and when the alarm is triggered Autoscaling will wait 300 seconds before allowing another scaling activity

Step scaling: in this case an alarm is attached to a scaling group. But the alarm settings and responses are defined in steps. For example, if the alarm is triggered when CPU is > 60 % and the alarm breach is at 70%, launch 2 instances, but if it's at 80% then launch directly 4 instances.