Learn in the textbookThe OSI Reference Model and Its 7 Layers
DefinitionThe application layer (アプリケーション層) is the topmost Layer 7; it provides communication services such as email and file transfer to the applications people use.
ExplanationEach purpose has its own protocol; for instance, viewing web pages uses HTTP, and sending email uses SMTP. It is the layer closest to the user, and data created here is passed down through the lower layers in turn before reaching the other side.
AnalogyIt is like the front counter of a shop that deals with customers directly. Orders still get delivered even if the counter staff know nothing about the shipping system behind them.
Memory hookAn application is the app you use every day. Remember this as the top layer, the one nearest the app.