File- Import Vs File- Open File

The only real significant difference with a file is in Multitrack view. If you import a file in multitrack, it will be imported into the files list, and that will be it - it's available to use, but nothing else has happened to it, so it's just sitting there. But if you open a file in Multitrack, then as well as placing it in the files list, it will be opened in Waveform view, and displayed there. So generally if you're working in Multitrack and you know that you just want to place clips that are already edited into a session, then use Import.

Obviously Import has other functions as well, mainly to do with data. But as far as files are concerned, that's it I believe.