Application-Layer AttackThis is not a specific attack but a grouping of attacks all using the same basic-principle (for more detailed information see the specific attack itself).
Generally: these type of attacks typically use a technique to compromise an application or Operating System to bypass security-measures and access controls.
Reasons for this type of attacks are generally: stealing (sensitive) data, phishing, system-sabotage and installing malware to make future attacks possible.
Some types of Application Layer Attacks are:
BGP hijacking, slowloris, show post, slow read, HTTP(S) flooding, DNS query flooding, low and slow attack, large payload post, Cache-busting attacks, WordPress XMLRPC flood attacks, GET/POST floods, attacks that target Apache webservers or the (Windows-)Operating system, OpenBSD vulnerabilities and mimicked user browsing.