A daemon is just an application. From the Wikipedia article.
In multitasking computer operating systems, a daemon (/ˈdeɪmən/ or /ˈdiːmən/)1 is a computer program that runs as a background process, rather than being under the direct control of an interactive user. Traditionally daemon names end with the letter d: for example, syslogd is the daemon that implements the system logging facility and sshd is a daemon that services incoming SSH connections.
I'll add the history of the term from that same page only because I find it amusing and interesting from a historical perspective.
The term was coined by the programmers of MIT's Project MAC. They took the name from Maxwell's demon, an imaginary being from a famous thought experiment that constantly works in the background, sorting molecules2. Unix systems inherited this terminology. Maxwell's Demon is consistent with Greek mythology's interpretation of a daemon as a supernatural being working in the background, with no particular bias towards good or evil. However, BSD and some of its derivatives have adopted a Christian daemon as their mascot rather than a Greek daemon.