In brief, CPU, NET and RAM are resources that the EOS network operates on.
CPU is the processing power of EOS. Essentially, it’s the amount of time a block producer should dedicate to processing your transactions. NET reflects the space on the network allocated for your transactions while they are being broadcasted to the blockchain.
After a transaction is successfully executed, the records of it will be stored on the EOS blockchain. This is where RAM comes into play. Information about your keys and balances also needs some space on the network, and the amount of RAM indicates your allowance for it.
Think back to every time you had to install an app — it usually takes a certain amount of time to do it, as well as pre-allocated space. And typically, the space it takes on the device differs from the one you see in the download manager, especially after you use it for a while. With this analogy in mind, imagine your device could only process downloads for a limited time — that would be your CPU resource, download ‘speed’ would correspondingly be NET, and the actual space an app takes on the drive — the RAM on your account.