System: is mostly where many users interact with each others (like dev.to or a hotel reservation)
App: where you use it alone (like Photoshop)
System: is like a factory where many people work to do a common goal (many services do lots of stuff to finish that room reservation the user wants)
App: that guy which works in that factory alone by himself and he is has mostly one single goal (Photoshop is mainly for creating that awesome design)
Windows / Linux / macOS are all operating "systems".
Voice recorder is an app.
MS Office word is an app.
Whatsapp provides an app for the end users to send and receive messages, and they also have a system where you send messages and they get stored / processed then encrypted in the middle and received from the other end and decrypted.
Now that you know the difference between a system and an app, @rhymes
already explained what a distributed system is...
You can see the addition on the word "systems" (in his comment) to make it "distributed": the components connect via network and talk to each others.
Just one more thing, mostly systems nowadays are a mere of SOA (Service Oriented Architecture).
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