It is a content delivery service. Content is replicated across different edge location. An edge location is not a region nor a AZ, it is a location where data is cached.
it caches static content (web distribution), dynamic, streaming (rtmp) data.
it well integrates with s3, ec2, elb, route53.
a origin for the CDN can be a ec2, s3 or a elb.
objs in CDN are cached with a TTL
manually cleared objs are charged by aws.
it supports integration with AWS WAF
it supports geo restrictions accesses