The biggest change in this is that now WildFly 8 is official Java EE7 Certified.
Undertow is new high performance web server written in Java. Now this has been implemented in WildFly 8. This is really designed for high throughput and scalability and can handle millions of connections. Undertow’s lifecycle is completely controlled by the embedding application. This is extremely lightweight with core jar having size of 1MB and embedded server using less than 4MB of heap space. This is really great.