Surprise! You have been using buffers all this time. A "buffer" is just "a file in memory"; if you have 1 split on your screen, with a different file in each split, then you are viewing 2 buffers.

The two things which will probably give you the biggest bang for your buck are:

  :ls

Which lists all buffers currently in memory, and:

  :b

Which supports tab completion, allowing you to instantly jump to any file with just a few keystrokes. You'll also probably enjoy Ctrl + 6 / Ctrl + ^, which swaps between two buffers.

You'll need to do

  :set hidden

before switching between buffers in a single window. Don't worry, though; if you try to quit without saving a modified buffer, or use :bd to delete a buffer without saving it, vim will yell at you.