A daemon is a process (program) that runs in the background on a multi-tasking operating system. This means that it is detached from a terminal and runs continuously in a non-interactive mode. You can create a daemon by forking a child process and then exiting the parent, which will cause it to be orphaned (on purpose) and adopted by the init process, the grandaddy of all processes in the system, (which is also a daemon itself). It is then left to run, waiting silently in the background until it is asked to do something.
There are many common daemons in use. One that you might have seen is the MAILER_DAEMON that routes and processes emails. If you misspelled an email address, you may get a bounce message from the daemon telling you it couldn't find it.Daemons
Another very common daemon is a web or file server. A web server runs continuously waiting faithfully to process http requests. When type a url into your browser, your computer might eventually connect to httpd, the http daemon, which will try to find the resource you're looking for and send it over to your computer via the http protocol. what is daemon and how is it work ?
Here's a pretty nice resource that explains basically how processes work and what a daemon is (and what it is not).
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A daemon process is essentially a program that runs in the background and is usually started when the operating system starts up. A typical daemon process in a mail daemon that runs in the background checking to see if you have received new mail and when you do it notifies you.
The opposite to a daemon process is a user process i.e. a program that is started by the user and stopped (killed) by the user e.g. the user starting a web browser.
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Difference between in a daemon, a process, and a service ?
Daemon - is a kind of process that runs in background as no association with terminal TTY or pts the example can be System V init etc.
Process - is a instance of an executable, for example a shell script or a command that you can run on background or foreground for performing some activities.
Services - it is a again another kind of process mostly not associated with terminal and runs in background to provide some services as Apache , x.initd which contains ftp, rsync etc. It basically to provide user a service as a server. Example Apache serves HTML containt to the users request, FTP provide file transfer services etc.
Many people confuse services and daemons. Services listen on ports. Daemons are a kind of process. Services can be daemons. Daemons don't need to be services.
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Service is something the operating system provides for all users. xinetd/inetd is one such example. xinetd takes care of popular services like ftpd.
A process is a running instance of an executable. It is represented by a process id and has an address space assigned to it.
Daemon is an application that has no terminal associations. One such example is init.
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