The tcpd program is a wrapper for network services that use the TCP protocol. It intercepts incoming connection requests and checks them against the rules specified in the /etc/hosts.allow and /etc/hosts.deny files. If the connection is allowed, tcpd executes the actual service program and passes the connection to it. If the connection is denied, tcpd logs the attempt and sends anerror message to the client. The tcpd program can be used to enhance the security and control of network access to various services, such as SSH, FTP, Telnet, etc.
The other programs listed are not directly related to the hosts.allow file, although they may be affected by it if they are wrapped by tcpd. The inetd and xinetd programs are super-servers that listen for incoming connections and launch the appropriate service program. The fingerd program is a service that provides information about users on a remote system. The mountd program is a service that handles NFS mount requests from clients. References:
* tcpd(8) - Linux man page
* Control server access using hosts.allow and hosts.deny files
* hosts.allow format and example on Linux