I am searching a lot for a definition about Farms, but I can't find one which fits my need.
Does anyone know a simple and easy way to explain it?
What is a Farm in SharePoint? How does it work?
You should check out the helpful posters about SharePoint Topologies, particularly the one about Microsoft SharePoint architecture.
The SharePoint farm is basically the lowest level in the topology, right after the server hardware. Let me explain:
You have one or multiple servers having SharePoint installed. The servers are connected through SharePoint and are all joined to the "SharePoint farm". The SharePoint farm could also just be one server, still it is the farm. There is only one farm per server, there cannot be multiple farms. Next to the servers running SharePoint itself, you also have one or multiple servers (cluster) running SQL Server holding the all data.
From the farm level you can go downwards in the topology (simplified):
I said "simplified" as I didn't go into the role IIS web applications play, content databases as well as service applications.