Your backpack is the single bag you begin with, and holds up to 16 different items. You can also acquire bags of various sizes for the four bag slots next to the backpack, but your backpack will always stay the same.
To open your backpack, you can either click on it, press the B key, or hit F12. You can open all bags at once by pressing Shift-B. Clicking on your backpack will also close all other open bags.